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A STUDY OF SOME NEUTRON-RICH ISOTOPES

OF LANTHANUM, CERIUM AND PRASEODYMIUM

BY MEANS OF THE FAST CHEMICAL ON-LINE

SEPARATION TECHNIQUE SISAK

GUNNAR SKARNEMARK

DEPARTMENT OF NUCLEAR CHEMISTRY

1977

r rA STUDY OF SCME NEUTRON-RICH ISOTOPES




GUNNAR SKARNEMARK

Department o f Nuclear Chemistry, Chalmers Univers i ty o f Technology,

Pack, S-402 20 Göteborg 5 , Sweden.

Dissertation

for the degree of Doctor of Technology (Teknologie doktorsexamen) in

Nuclear Chemistry (examiner: Professor Jan Rydberg), by due permission

of the School of Chemical Engineering at Chalmers University of

Technology to be publicly discussed in lecture hall KD, Kemigarden 3

on Saturday the 14th of May, 1977, 10.15 a.m.

Faculty opponent: Professor Alexis C. Pappas, Department of Nuclear

Chemistry, University of Oslo, Oslo, Norway.

rERRATA

Co the thesis

1977-05-1*

A STUDV OF SOME NEUTRON-RICH ISOTOPES OF LANTHANUM, CERIUM ANO PRASEODYMIUM BY MEANS OF THE FAST

CHEMICAL ON-LINE SEPARATION TECHNIQUE SISAK

Locat ion

p.13. footnote to Table 2.1.

p.13, sect. 2.2.1, line 5

p.13, sect. 2.2.1, line 6

8

by Gunnar Skarnemark

Is written Should be

To be added: The chemical yields and the growth from La and Ce

precursors have not been taken into account.

15 cm3 10 cm3

p.18, line 7

p.18, line 10

p.20. Table 2.3

p.22, sect. 2.4, l ine 4

p.23, line 3

p.23, line 8

p.23, line 3 from bottom

p.24, line 19


p.30, line 12

p.30, line 14

p.30, last line

p.31, line 5

p.32, sect. 3.3, l ine 7

p.36. sect. 3.4, l ine 8

p.39. line 1

p.39, line 9

p.40, line 11

p.41, caption to Fig. 3.12, line 2

p.44, l ine 3

p.44, l i ne 13

p.44, l ine 17


p.43, line 9

p.48, sect. 3.6, line 4


p.50, last line

p.52, line 6

p.53, sect. 3.8, line 1

p.55, sect. 4.2.2, line 7

p.59, line 2

p.60, line 20

p.62, line 9

p.62, line 14

p.62, line 19

p.67, Fig. 6 . 1 , in the upper

spectrum

p.67, l ine 4



p.72, line 6


p.74, caption to Fig. 6.5.

sum of the spectra was

spectra

To be added below: The same values

Compu*

disc- /pi cal

extraction conditioning

och

performed

GJRT

duster

duster

KeV

3.6.Rao 6-

solutionsCe

'<o>0solution

Ce tracer

count rate

[Hof 66]

column, in the SISAK experiments

i s , that

s t ra î gh t

act iv i ty

132Nd

ments, on

2.2 a

ran

i .e . teflon

measurements

[IX]

FMHM

sections

seem

433.1 La-H*

1 = 5/2

greater

Before discussion

0.7.level is

7.1

sums of the spectra were

spectrum

were obtained for toluene and xylene

computer

discuss a typical

extraction, back-extraction and

condi tioning

and

preformed

GJRT technique

cluster

cluster

keV

3-8.Rao 66solution

Ce (IV)

Mox)

solutions

C r t race r

countrate

fHo< '.$]

column in the SISAK experiments.

is that

straight-

activit ies52Nd

ments on

2.2 s

has run

i .e- a teflon

measurement

[HI]FWHM

chapters

seen

433.1 La-1A3

I - 5/2

larger

Before the discussion

0.7 s.level at 938.8 keV is

6.1

J

p.78, line 16

p.81, sect. 6.4, line 2

p.83, sect. 6.4.2, line 12

p.83, last line

p.84, Fig. 6.11

p.84. Fig. 6.11

p.85, sect. 6.4.3, line 16

p.86, Fig. 6.13


p.93, line 5

p.93, line 17

p.96, sect. 7.1.1, last line

p.99, line 14

p.99, line 16

p.106, sect. 7.4.2, line 1

p.106, sect. 7.4.2, line 2

p.109, line 6

p.109, lines 11, 12

p. 112, line 7 from bottom

p.113, Fig. 8.Z, to the right

p.114, sect. 8.3.1, line 2

p.116, caption to Fig. 8.5. line 2

p.119, line 7

p.123, line 5

p.124, sect. 8.5.2, line 7

p.124, sect. 8.5.2, line 12

p.124, sect. 8.5.2. line 18

p.126, Fig. 8.8, to the right

p.126, line 5

p.126, line 6



p.131, ref. CBor 71]

Paper [I], p.1695, ref. 8

Paper [I], p.1695, ref. 11

Paper til], p.2399, Fig. 3

Paper [II], p.2400, sect. "Two-

detector delay...", line 13

Paper [IV], p.755, sect.

EXPERIMENTAL, line 10

Paper ÏIV], p.761, Table 4,

last line

Paper [VII], p.101, Fig. 4

Paper [VIII], p.4, sect. "Decay

scheme", line 3

Paper [VIII], p.5, sect. "Half-

life"

Paper [VI11], p.6, line 2

Paper [VIII], p.6, sect. "Half-

life", line 3

ragreeennt agreement

spontanuous spontaneous

6.14. 6.13.

orginate originate• < 666.4 keV « * 666.4 keVTo be added to the caption: The ha l f - l i ves l is ted were obtained a f te r

subtraction of the longer-lived component.

6.14 6.15

There should be a coincidence relationship between the 503-2 and~3B0.I

keV yrays and the 515-0 and 36ß-3 keV y-rays, respactively-

[Bow 71 , Che 71, Nuc 73. Kal 75]

f i g . 6 . 1 8 . , the strongest La T"ray

tf 6.4.original lyinterceptsoriginating7-7-7-6.

[Bow 70, Che 71 , Nuc 73]

f i g . 6 .17 - , the l l | 8La Y-rays

table 6.6.orginallyinterceptorgi nating7.6.

' * V . p > 1Il6NdPin 76, Pin 77a, Pin 77b

then

3 2unatnbiguons

[Hue 73, Lie 74]

Pin 76a, Pin 76b

them

3/2

unambiguous

[Nuc 73, Lie 74, Kal 75]

The sentence "The 2 s t a t e . . . " should be omitted

that a

orginating

and [Sey 73].

7.7.

93i 3+

probably

8.10.

Bow 70

avknowledgements

Borg, S . , Rydberg, B.,

H.F.O. Lawrence

Borg, S . , Rydberg, 8 . ,

that the

originat ing

and Seyb [Sey 73].

7-ft

931 3~

probable

8.9.Bow 71

acknowledgements

Borg, S . , Bergström, I - , Rydberg, 8 . ,

F.O. Lawrence

Borg, S . , DergstrSm, I . , Rydberg, B.,

Error bars drawn ar^ the re la t ive errors taken as absolute errors

detector 1 and 2

0.05 M HjïO

detector 2 and 1

0.05 H H202

1365.6 1356.6

The 156.0 keV Y-ray should be r<rawn from the 321.0 keV to the 165.0

keV level

Fig. 3-

Fig. 4.(Fig. 5)

Fig. 6.

Fig. 5.

Fig. 3.(Fig. 6)

Fig. 4.

_Î2E---Txr

- 4*

2*

MU.«s

3S1

130

712 _ («•) 71« _ •

4- 404

122 2;

0' O 0* 0 0* O 0- O

M«C. «2s™ '54G.I 15«o,

This figure replaces the lower part of Fi9. 6.20. (the N - «0 level systenatics).

rA STUDY OF SOME NEUTRON-RICH ISOTOPES




GUNNAR SKARNEMARK

Department o f Nuclear Chemistry, Chalmers Univers i ty o f Technology,

Fack, S-402 20 Göteborg 5 , Sweden.

This thesis includes the fo l lowing pub l ica t ions , which w i l l be

re fer red to by roman numerals I - I X .

I P.O. Aronsson, G. Skarnemark and M. Skarestad. Snort -1 ived Isotopes

o f Lanthanum, Cerium and Praseodymium Studied by SISAK Technique.

J . Inorg. Nucl . Chem. #» ( 197 »> 1689

II P.O. Aronsson, B.E. Johansson, J . Rydberg, G. Skarnemark, J . Alstad,

B. Bergersen, E. Kvâle and M. Skarestad. SISAK - A New Technique

for Rapid, Continuous (Radio)chemical Separations. J . Inorg. Nucl.

Chem. 36 (197*) 2397

I I I P.O. Aronsson, G. Skarnemark and M. Skarestad. The Ha l f - l i fe of

Ce Obtained by SISAK Technique. Inorg. Nucl. Chem. Letters _H)

0974) 499

IV P.O. Aronsson, G. Skarnemark, E. Kvâle and M. Skarestad. Decay

Characteristics of Some Neutron-rich Lan thanide Nuclides Obtained

by SISAK Technique. Inorg. Nucl. Chem. Letters JO (1974) 753

V N. Trautmann, P.O. Aronsson, T. Björns tad, N. Kaf f re l l , E. Kvâle,

M. Skarestad, G. Skarnemark and E. Stender. The Combination of

the Gas Jet Recoil Technique with the Fast Chemical On-line Sepa-

ration System SISAK. Inorg. Nucl. Chem. Letters JJ (1975) 729

VI T. Björnstad, E. Kvâle, G. Skarnemark, P.O. Aronsson, N. Kaffre l l ,143N. Trautmann and E. Stender. Decay Properties of La. J . Inorg.

Nucl. Chem. In print

J

VII G. Skarnemark, E. Stender, N. Traut mann, P.O. Aronsson, T. Björn-

stad, N. Kaf f re l i , E. Kvâle and M. Skarestad. Decay Properties

of Some Neutron-rich Praseodymium Isotopes. Radiochim. Acta 23

(1976) 98

VI I I G. Skarnemark, P.O. Aronsson, T. Björnstad, E. Kvâle, N. Kaf f re i l ,

E. Stender and N. Trautmann. Decay Properties of % |_a.

J . Inorg. Nucl. Chem. In print

IX T. Björnstad, E. Kvâle, G. Skarnemark and P.O. Aronsson. Decay

Properties of Some Neutron-rich Cerium Isotopes. Submitted to

J . Inorg. Nuci. Chem.

2. J

CONTENTS

ABSTRACT

1. INTRODUCTION

2; SISAK EQUIPMENT

2.1 Irradiation facilities

2.2 Target

2.2.1 The ion exchange target

2.2.2 The gas jet target

2.3 Separation equipment

2.3.1 The degassing unit

2.3.2. The SISAK wet chemistry equipment

2.4 Nuclear detection equipment

3. SISAK CHEMISTRY

3.1 General demands on the SISAK chemistry

3.2 Transfer of radioactive species from the gas jet

system to a liquid phase

3.3 The decontamination step

3.4 The Ce system

3.4.1 The oxidation and extraction of Ce

3.4.2 The reduction and back-extraction of Ce

3.4.3 The extraction chromatography column

3.4.4 Some operational characteristics of the Ce system

3.5 The Pr system

3.5.1 Some operational characteristics of the Pr system

3.6 The La system

3.7

3.6.1 Some operational characteristics of the143The La system

8La system

1433.7.1 Some operational characteristics of the •'La system3.8 The conditioning step

4. DATA AQUISITION METHODS

4.1 Collection of y-ray singles spectra

4.2 Half-life determinations

4.2.1 Measurements according to the conventional technique

4.2.2 Measurements according to the TDD technique

4.3 Y~Y coincidence measurements

5. DATA EVALUATION METHODS

5.1 Evaluation of Y~ray singles spectra

5.2 Treatment of half-life data

6

7

10

10

13

13

13

15

15

15

22

23

24

26

32

36

37

41

43

45

46

47

48

50

50

52

53

54

54

54

54

55

56

58

58

59

F

6.

7.

5.3 Evaluation of T'Y coincidence data

5.4 Calculation of relative y ray intensities

DATA OBTAINED FOR THE DECAY OF NEUTRON-RICH La ISOTOPES

6.1

6.2

6.3

6.4

6.5

6.6

143Data obtained for the decay of La

6.1.1 Mass assignment

6.1.2 Half-life

6.1.3 r~ray data and decay scheme144

Data obtained for the decay of La


6.2.2 Half-life

6.2.3 vray data and decay scheme



6.3.2 Half-life

6.3.3 Y-ray data and decay scheme



6.4.2 Half-life


Data obtained for the decay of


6.5.2 Half-life and Y-ray data

1*7,La

148,LaData obtained for the decay of


6.6.2 Half-life and Y-ray data

DATA OBTAINED FOR THE DECAY OF NEUTRON-RICH Ce ISOTOPES7.1

7.2

7.3

lie

Data obtained for the decay of Ce


7.1.2 Half-life


Data obtained for the decay of Ce


Data obtained for the decay of 1<l7Ce and 1<>8Ce

7.3.1 Mass assignments

7.3.2 Half-lives


60

62

64

64

65

66

66

70

71

71

72

78

79

79

81

81

82

83

85

89

89

90

92

92

92

95

95

96

96

96

100

100

102

102

103

103

4.

r r149 1507.4 Data obtained for the decay of 'Ce and Ce

7.4.1 Mass assignments

7.4.2 Half-lives7.4.3 Y-ray data

8. DATA OBTAINED FOR THE DECAY OF NEUTRON-RICH Pr;ISOTOPES

8.1 Data obtained for the decay of 1 Pr147

8.2 Data obtained for the decay of 'Pr


8.2.2 Half-life


8.3 Data obtained for the decay of Pr


8.3.2 Half-life


8.4 Data obtained for the decay of Pr


8.4.2 Half-life

8.4.3 Y-ray data and.decay scheme

8.5 Data obtained for the decay of ' Pr


8.5.2 Half-life


9. FUTURE INVESTIGATIONS OF THE HEAVY La, Ce AND Pr ISOTOPES

10. ACKNOWLEDGEMENTS

REFERENCES

ABBREVIATIONS

105

105106

107

109

109

110

110

110

110

114

114

114

115

119

119

120

120

123

123

124

124

128

129

131138

5. J

rABSTRACT

The fast on- l ine chemical separation technique SISAK has been u t i l i z e d

to study the decay propert ies o f neutron-r ich isotopes of La, Ce and Pr.

The resul ts include pa r t i a l decay schemes and r~ray in tens i ty data for

14 min TZ|3La, 42 s i W L a , 25 s 1 I | 5La, 9 s 1 I | 6La, 3 min ^5Ce, Hi min1 l |6Ce. 56 s ^ C e , 50 s ""»Ce, 12 min 1<>7Pr, 2 min i W P r , 3 min *

and 6 s 150„Pr. Half-lives and Y**ray energies are reported for thepreviously unknown nucl ides 'La (T * 2 2 s)

(T

'La (T, . , * 2.2 s) ,

5.7 s) and i:>uCe (T, .- - *

La (T, . , ~1 s ) ,1 s). The nucl ides were formed

in thermal neutron-induced fission of -"U. The fission products weretransferred to the SISAK system via a gas jet recoil transportation(GJRT) system.

The combination of the GJRT system with SISAK is discussed, as wellas the chemical separation systems used for the isolation of La, Ceand Pr.

J

I . INTRODUCTION

Though fission products have been studied for almost *»0 years, there

are s t i l l parts of the nuclide chart where the decay properties of short-

lived nuclides are known insufficiently or not at a l l . Considerable

efforts have been devoted to the development of selective methods to

separate and investigate these nuclides. For some elements l ike noble

gases and volati le elements, on-line mass separation methods have been

successful, while for non-volatile elements l ike those around A = 100

and the lanthanides chemical methods are the most convenient.

To contribute to the methods available for production of samples

intended for nuclear spectroscopic studies, the on-line chemical sepa-* *

ration technique SISAK was developed. This technique is described

in paper [ I I ] and ref. [Aro 7*»]. SISAK is intended to continuously de-

liver radiochemical ly pure ( i . e . only one element present) samples of

nuclides with half- l ives down to 0.5 s. By this technique, the decay

characteristics of the neutron-rich light lanthanides La, Ce and Pr

have been carefully investigated.

Why is i t of importance to study these nuclei? To answer this ques-

tion is simple: being situated in the shape transition region between

spherical and deformed nuclei, these light lanthanides are of great

theoretical interest. Information about the nuclear properties in this

region may be obtained only from a detailed knowledge of the decay sys-

tematics of these nuclides. However, the information available about

neutron-rich La, Ce and Pr isotopes was, at the time when the SISAK

investigation began, 1 i mi ted to some half- l ives and a few y- ray energies.

There are several reasons why these elements had been so sparingly

studied, e.g. the short ia l f - l ives , which make most of them inaccessible

for conventional o f f - l ine chemistry and the low vola t i l i ty which makes

them impossible to study by most mass separators. The only lanthanides

accessible for studies with ISOL-facilities have been those with A<146

since these mass chains are released as the 1 an than i de precursors Cs

and Ba. For ," > •%, the cumulative fission yield of these volati le

elements is low and thus the corresponding lanthanides are inaccessible.

Some investigations of neutron-rich La, Ce and Pr isotopes have been

In this thesis, "short-lived" means oa l f - l i f e less than a few minutes.* *

"SISAK » Short-lived Isotopes Studied by the AKufve technique

rF

performed using the fast , automatized of f - l ine chemical separation tech-

nique developed in Mainz [Seh G9]. These studies yielded valuable know-

ledge about half- l ives and r~ray energies of these nuclides [Sey 73• Fis

74]. The short half- l ives in combination with low fission yields and weak

y-rays required, however, numerous repetitions of the experiments. This

made the technique less suited for coincidence measurements in this mass

region.

WMhelmy [Wil 69] measured mixed fission products (including La, Ce

and Pr) collected on a moving tape without mass or chemical separation. The

low selectivity of this method makes many of the results less accurate

due to d i f f icul t ies in the analysis of the extremely complex T-ray data.

Data on La, Ce and Pr isotopes have also been reported by authors

studying prompt Y-rays from fission fragments formed in the spontaneous

fission of 2 5 2Cf [Che 7 ' ] . These data usually include only transitions

belonging to the ground state rotational bands in even-even nuclei.

Data on other nuclei are mostly less accurate.

Recently, the actual nuclides have been studied by the I SOL-technique

at the LOHENGRIN mass separator in Grenoble [Dev 76]. This mass separa-

tor does not, as most separators, include any ion source in which the

nuclides studied have to be volat i l ized. Instead i t ut i l izes the charge

and kinetic energy given to the fragments in the fission process. The

separation is achieved in crossed electr ic and magnetic f ie lds. The

results obtained include hal f - l ives, Y" ray data and a few decay schemes.

This thesis describes and discusses data obtained for neutron-rich

La, Ce and Pr nuclides by employing the SISAK technique. The experiments,

which took place in the years 1972 - 1976 were performed in a jo int

Swedish-Norwegian-German group ("the SISAK Collaboration") from the

Departments of Nuclear Chemistry at Chalmers University of Technology,

Göteborg, the University of Oslo and the Institut für Kernchemie, Jo-

hannes Gutenberg Universität, Mainz.

The nuclear data presented in this thesis include ha l f - l ives , Y~ray

energies and relative intensities for U 3 " l 4 8 L a , 1*5-150^ „ d _l*7H50^r.

altogether 16 nuclides with half- l ives between 14 min ( H 3 L a , i 46Ce)

and I s ( La). Partial decay schemes are proposed for 13 of these

nuclides. Where possible, the spin and parity of levels are discussed,

especially for even-even nuclei. The SISAK results are also compared

to other available results. The choice of chemical separation systems

J

used for the isolation of La, Ce and Pr is discussed, as well as the

performance of the systems. A discussion of the target systems used

is also included.

The thesis includes the data presented in the papers [I - IX], butpresents also new and unpublished results, especially for the mostshort-Iived nuclides.

9.

2. SISAK EQUIPMENT

2 . 1 . Irradiation fac i l i t i es

Before discussing the equipment used in the experiments, i t is

necessary to give a brief summary of the irradiation fac i l i t ies

ployed.

The f i r s t attempts to study the l ight , neutron-rich Ian than ides by

SISAK technique were made at the 14 MeV neutron generator of the De-

partment of Nuclear Chemistry at Chalmers University of Technology in

Göteborg. The flux of this neutron source was, however, too low (~IO

n cm s at the tube face at 100 kV voltage) and too tine-dependent

(the target h a l f - l i f e was less than 2 h) to allow any sophisticated

experiments. Furthermore, at that time the SISAK technique was in a

"pre-SISAK" stage, so these runs must be regarded only as test experi-

ments.

In 1972, the SISAK 1 system was completed, and to get an acceptable

neutron flux we installed i t at the 14 MeV neutron generator of the

Department of Nuclear Chemistry, university of Oslo. This neutron gene-

rator features a sealed-off accelerating tube with a self-replenishing

trit ium target. The neutron flux at the tube face is ~3«10 n cm s at

150 kV voltage.

Compared to the runs at the Göteborg neutron generator, the expe-

riments in Oslo were a great step forward, not only with respect to

the higher flux but also the possibility of running the experiments

continuously for hours or even days. The work in Oslo was successful,

although we s t i l l fought against low sample strengths and a severe

N contamination due to the target arrangement used (cf. section 2 .2 .1) .

The Oslo experiments led to the results published in the papers [ I - IV] .

In 1974, the Oslo neutron generator broke down, and facing the possi-

b i l i t y of an interruption of almost a year, we moved the SISAK equip-

ment to the Institut für Kernchemie, University of Mainz, where i t was

installed at the Cockcroft-Walton accelerator, which yields a 14 HeV

neutron flux (at the tube face) of more than 3-10 n cm s~ at 400 kV

voltage. The target h a l f - l i f e is ~10 h, thus also allowing comparatively

long runs. With this accelerator we got strong samples of the longer-

lived nuclides (T j - 2 > 20 s ) , while the short-lived nuclides were

10.

suppressed because of the long liquid transport tine through the con-crete shielding surrounding the accelerator (the transport distancefrom target to detector was ~I2 m compared to 2.5 m in Oslo; a differ-ence of ~J5 s).

• n 1975, the system was installed at the gas jet recoil transporta-tion facility in the Mainz TRIGA reactor, which reduced the transporttime (the different parts of the system could be put very close to-gether, which yielded short transport times). The intensity of theseparated samples also increased.

Finally, in 1976 we installed the new SISAK 2 system at the MainzTRIGA reactor. This Is the fastest set-up so far; the most short-livedI anthanide studied is ~l ssection 6.6).

148, La (this nuclide wi l l be discussed in

The present experimental configuration, used in the SISAK 2 experi-

ments in Mainz, is shown schematically in f ig . 2 . 1 .

figure 2.1. General plan of the SISAK experimental area at the Mainz TRIG» reactor.I - gas jet control, 2 « reactor shielding, 3 ' TRIG» core, * - target, 5 - transport capillary.6 - plug filled with boric acid paraffine, 7 - degassing unit, 8 - centrifuges, 9 • detectors,10 - control panel, II - storage vessels, 12 - thermostatic baths, 13 - heat exchangers, I* - wastecontainers, 15 - detector shielding (concrete), 16 - detector electronics, 17 - frequency converters.

The figure is not drawn according to scale (the reactor is underdimensioned).

II.

rThe experiments performed in Mainz have so far resulted in the

papers [V - IX].

A comparison of the transmission of some long-, medium- and short-

lived nuclides through the different Oslo and Mainz configurations is

given in table 2 . 1 . In this table i t is easily seen that the SISAK 2

equipment favours the most short-lived nuclides. This effect is demon-

strated by the spectra shown in f i g . 2.2. They have been recorded using

the SiSAK 1 and SISAK 2 equipments at the TRIGA reactor.

F'1V» *•?• V'«ir »*ctr. mntuni « th. n.inz TM« rwctor « I n , the SISNC 1 mi SIS« 2•^uipamts m i l

12. J

Table 2.1. The transmission of some nuclides through the different SISAK configurations used in

Oslo and Hainz.

ExpcrlaenE site

SIS« 1:

Oslo

Hainz CockcroFt-Walton

Hain« TMCA

SI3KI!

Halm TAIG»

Target

3 9 2J*U

3 g " » u

1 ag 235U

1 ag M 5U

«eutron flu«, n » S

J-t»' (Id NeV)

J-101* (III HeV)

lO11 (theraal)

lo" (theraal)

T J / 2 - l * ain (

0.99

0.98

0.99

0.99

Transaission

"*Ce) 3 ain ( l l l5Ce)

0.S6

0.S3

0.57

O.SS

«s <'*\.|

».M

«.73

a.n

o.s*

1 »s <"*U)

•.50

•.28

• . M

•.79

«s (I5iC«l

•.15

•••3

«.25

».51

2s (""la)

•.•2

• . M

•••»

«.25

The transmission is defined as the ratio between the nurter of ato-w/s passing the detector site and

the number of atoms/s released fron the target.

2.2. Target

2i2i2t_The_ign_exçhange_target

In Oslo and at the Main? Cockcrcft-Walton accelerator, we used as

target approximately 3 grams of * U adsorbed on an an ion exchanger

(Oowex 1x8, 2-400 mesh). This ion exchange resin was contained in a

titanium cell approximately 30 cm3 in volume ( f i l l ed with ion exchanger,

the free liquid volume was about 15 cm assuming ~30 % void fraction

in the ion exchange resin). When eluting the resin with 0.01 H NH_S0.,

only Kr, Xe, Rb, Cs, Sr, Ba, Y and the I an than ides were released.

Other elements, l ike Br, I , 2r , Nb, Ho, Tc, Sn and Sb were so strongly

adsorbed, that they could be seen only as very weak contaminants. The

strongest contaminant when usinq this type of target was 16N from the

reaction 0(n,p) N. This nuclide has three high energy Y-rays yielding

an enormous Compton distribution below ~7 MeV. The problem with the I 6N

contamination was never solved completely, although we decreased the

N extraction by using organic solvents in which water has a low so-

lubi l i ty ( N is mainly extracted together with water).

The ion exchange target has been discussed thoroughly in refs.

[ I I , Aro 74] and wi l l not be further discussed here.

Gas jet targets have now been described by many authors [Ghi 59,Mac 69, Bow 72, Wie 72, Dau 73, Kos 73, Puu 73, Wie 73, Aum 74b, Hel 75,Kos 7k, Sys 7k, Kos 75> Sch 75] in neutron as well as charged particleirradiations. The I a r get container used by us is in principle construc-

13.

r

Figur« 2.3. The 9*5 je t target cell (thenHlization chanter) used in Mainz. The drawing isfro» ref. (Ste 7*].

ted l ike most other gas j e t target cells and is shown in f ig 2.3- I t

consists of a thermal izat i on chamber made of A I , where the fission

products recoiling out of the target become attached to small ethylene

droplets ("clusters") dispersed in N_ (rat io I : 1.4 total pressure

1500 torr) or to C02 clusters (in C02 gas of 1500 t o r r ) . The target

is either 2 3 5 U , 239Pu or 2*9Cf molecular-plated on to an AI backing,

using the procedure published by Aumann et a l . [Aum 74a]. The thickness

of the target element layer is less than the range of the fission frag-235merits. The targets used so far have been either 400 or 1000 yg *"J"'U0,

2 39 249400 vg PuO2 or 10 yg C^0?- The Cf target was only used for atest run in which i t proved to be too small to allow studies of neutron-

249rich lanthanides. The Z (most probable charge) values of Cf in the235I an than i de region were also unfavourable compared to those of U.

249However, Cf w i l l be a useful target when studying elements in the

mass range 105 -120.

The hold-up volume of the target cell used in Mainz is approxima-

tely 12 crrr yielding a mean hold-up time of 0.6 s at 20 ml s gas flow

(at 1500 t o r r ) . The total delay of the gas j e t target including passage

through 7 m of capillary (polyethylene tubing; inner diameter 1 mm) is

~0.7 s iTra 75]. Thus the system provides a very fast means of trans-

portation between the reactor and the SISAK system.

The thermal izat i on chamber is mounted at the end of a plug, which

is inserted into one of the four beam holes of the Mainz TRIGA reactor.

Usually, the position yielding the highest neutron flux (~10 n cm s )

has been ut i l i zed .

The gas je t transportation system used in Mainz is described in more

detail in réf. [SiI 77].

14.

2.3 . Separation equipment

Since the SISAK equipment is a wet chemistry system, i t must beconnected to the gas je t arrangement via some device in which the radio-active species are transferred from the clusters carried by the inertje t gas to a l iquid phase (aqueous or organic). For this purpose weuse a stat ic mixer (Kenics Corp., USA) where the gas is extensivelymixed with a l iquid phase, usually a n i t r i c acid solution (1 M orpH ~1.4) preheated to approximately 90°C by heat exchange. The transferprocess w i l l be discussed in more detail below (section 3.2.).

In principle, i t would then be possible to feed the gas-liquid mix-ture directly into the f i r s t mixer-centrifugal separator unit thususing the centrifuge as a degasser. However, this procedure is not re-commended, because i t affects the behaviour of the centrifuge ( i t ismore d i f f i cu l t to obtain good phase purity) as well as the radiochemicalpurity of the outgoing organic phase (the noble gases are extracted).

In order to avoid these inconveniences, the gas-liquid mixture leavingthe stat ic mixer is fed into a degassing unit consisting of a vertica"glass tube with an inner diameter of approximately 70 mm. The heightof this simple cyclone is about 150 mm. Inside the tube, there arebaffles, to break the rotation of the l iquid, which is injected tan-gential ly. The conical bottom of the degasser (the l iquid outlet) isconnected to the f i r s t centrifuge stap of the chemical separation sys-tem via a pump; the upper (gas) outlet is connected to the suctionsystem of the reactor ha l l , so that the noble gases and other volat i leproducts swept of f do not contaminate the air in the ha l l .

The chemical problems in connection with the transfer of the radio-

active species from the clusters to the l iquid, as well as the chemistry

inside the reaction chamber and the choice of gas w i l l be discussed

in section 3.2.

The equipment so far discussed is constructed in the same mannerin the SISAK I äs well as the SISAK 2 system. In other respects, however,the systems arre, not identical and therefore the two systems used forthis investigation w i l l be described separately.

15.

The SISAK I system has already been described in refs. [ I I ] and

[Aro 74] , but nevertheless i t is important to give a brief presenta-

tion of the system here.

Its main components are k centrifuges, I I pumps and a control panel

(including storage vessels for the solutions). The centrifuges are of

the H-32 type (MEAB Metallextraktion AB, Göteborg) [Rei 73] with a

liquid hold-up volume of ~100 cm .

The maximum flow-rate through such a centrifuge is 15"I7 ml s per

phase at the normal running speed of 12-18000 rpm. This yields a mean

hold-up time of ~3 s. The centrifuges are made of titanium and driven

by 800 W 3-phase asynchronous electr ic motors regulated by a continu-

ously variable (0 - 300 Hz) AC frequency. This AC is supplied by f re -

quency converters (manufactured by b'berg Machine Company, Eskilstuna)

of the rotating type, i .e . an AC generator driven by a DC motor.

The unique feature of the H-centrifuge is the abi l i ty of absolute

phase separation, i .e . none of the phases contains more of the opposite

phase than that corresponding to the solubil i ty.

To obtain adequate mixing of the two phases before they enter the

centrifuge, they are mixed in a static mixer of the same types as that

connected to the degassing unit and are then fed into a funnel connected

to the centrifuge in let . Early attempts to connect the mixer directly

to the inlet did not yield pure phases [Aro 77] , and so the funnel was

included although i t reduces the rapidity of the system sl ightly.

The centrifuge outlets are equipped with pressure meter/throttle

valve units allowing both remote pressure reading and throttle valve

control. These valves are necessary to maintain a suitable pressure

on the outgoing phases (to obtain absolute phase puri ty) .

AH pumps used in the systems are of the cog-wheel type and made of

titanium with cog-wheels of kel-F (polytrifluorochloroethylene; manu-

factured by Fluorocarbon Co., USA) They are lubricated by the solution

pumped and driven by DC -rotors. The flow-rate through such a pump is

continuously variable between 0 and ~30 ml s" 1 . The maximum pressure

capacity of the pumps is ~7 atm.

The whole system is controlled from a central control panel which

can be placed up to 15 m (with the cables used) away from the wet

chemistry system. In addition to the controls and instruments for

16. J

the pumps, the pressure meter, the valves, and the centrifuges, the

panel is also equipped with flow meters (Rota AG, Germany) and an

instrument for remote temperature reading.

The total transport time through the system (with 3 centrifuges in

use) is indicated in table 2.2. So far , 4 s Ce is the shortest half-

l i f e studied with the SISAK I system. Before the description of SISAK 2 ,

i t should also be mentioned that SISAK I has been used in more than

80 % of the experiments covered by this thesis.

Since the spring of 1976, we use a new, improved system called

SISAK 2. I t has basically similar working principles to SISAK I (the

degasser and the pumps are, e . g . , identically constructed), but there

are certain differences as discussed below.

The great improvement in SISAK 2 compared to SISAK I , is the new

H-IO centrifuge (MEAB Metallextraktion AB, Sweden), which is a scaled

down version of the previously-mentioned H-32 centrifuge. The hold-up

volume of the H-10 centrifuge is only 12 ml, while the maximum through-

put (in good chemical systems) has been measured to be as high as 23 ml

s per phase (corresponding to a mean hold-up time of ~0.25 s ) . under

practical running conditions, flows of 15-20 ml s" are easily obtained

(corresponding to 0.40-0.30 s hold-up time). The use of the H-10 cen-

trifuge decreases the shortest h a l f - l i f e l imit for the nuclides accessible

with the SISAK technique from ~3 s to ~0.5 s. The transport time through

a 3-centrifuge SISAK 2 set-up is shown in table 2.2.

Table 2.2. Approximate mean transport times through SISAK 1 and SISAK 2 set-ups involving threecentrifuges. The figures were calculated assuming a flowrate of 15 ml s per phase in the SISAK 1system and 20 ml s* per phase in the SISAK 2 system.

Transport Transport time, s (SISAK 1) Transport time, s (SISAK 2)

From targetto degasser

Through degasser

Oegasser to CI

Through CI

CI to C2

Through C2

C2 to C3

Through C3

C3 to detector

Total, target todetector

0 7*

1

0.4~30.4~30.4~31.S

-13.5

0.7

1

0.3

0.4

<0.l

0.1)

<0.1

0.4

1.5

17.

rTo investigate the delay properties of the SISAK 2 system, we have

performed a series of experiments in which the TRIGA reactor was opera-

ted in the pulse mode. The Y-ray measurements, which were performed in

different positions (see f i g . 2.4) were started 1-3 s after the pulse

maximum, depending on the number of centrifuges involved. The measure-

ment included 32 x 0.5 K spectra, usually measured for 0.5 s (sometimes

0.3 s ) . The integral sum of the spectra was plotted (after dead time

correction) versus time. Such curves, valid for the degasser alone

and the one, two and three centrifuge stages are shown in f i g . 2.4.

To use the integral sum of the spectra is not the most convenient

method because growth-and-decay phenomena might disturb the shape of

the transmission curve, but the counting statist ics did not allow

single Y~ray peak areas to be used.

Flaure 2.t. Curves showing the transport of the activity released in a TRIGA pulse from thegas jet target to different positions in the SISAK 2 system. The experimental set-up and themeasuring positions are shown in the flow diagram. The curves are valid for 20 ml s*1 per phaseflowrate.

18. J

The H-10 centrifuge is basically a linear scale-down of the larger

models [Per 76], but there are some differences, e.g. that the inlet

and outlet channels are relatively wider to accomodate the high flow-

rates. The motor is also over-dimensioned in order to achieve a more

rapid speed control.

To obtain thorough knowledge of the behaviour of the H-10 centrifuge,

i t was carefully tested with the most common organic solvents. The

aqueous phases tested were limited to NaCIO. solutions of different

concentration (0 - 5 M) because the tests were performed with a brass

prototype of the centrifuge (the final version is made of titanium).

For most of the systems studied, the centrifuge showed a good per-

formance [Sel 75], but i t proved to be much more sensitive to e.g.

flowrate changes than the larger centrifuge. This sensitivity makes

i t somewhat more d i f f i cu l t to obtain pure phases. Probably, this depends

on a much shorter distance between the phase boundary and the outlet

channels. For this reason, the SISAK 2 system is somewhat more d i f f i cu l t

to run than the SISAK I , but for an experienced operator i t presents

no problem. Once the system is running with pure phases, i t is as stable

as SISAK I .

The results of the H-10 tests are summarized in table 2.3 and f i g .

2.5. When looking at these figures, i t must be remembered that the

tests on which table 2.3 is based were performed with the prototype

centrifuge equipped with an airturbine motor (maximum speed ~14000 rpm),

while f ig . 2.5 is based on a run with a centrifuge driven by the 400 W

asynchronous electr ic motor f inal ly chosen.

To obtain the results shown in f ig . 2.5, a solid state frequency

converter that allowed a speed up to 80000 rpm was used. The converters •

f inal ly chosen for the SISAK 2 equipment allow the centrifuges to be run

from 2000 up to 25000 rpm ( i . e . they deliver an AC frequency continuously

variable between ~30 and ~400 Hz). The H-10 centrifuge wi l l be further

described elsewhere [Ska 77a].

In our application of the H-10 centrifuge, i t is equipped with a

static mixer (the hold-up volume of which is only 50 % of the mixers

used in the SISAK I system) connected directly to, the inlet of the

centrifuge. As was mentioned before, this was not possible with the old

centrifuge. This difference in the performance of the centrifuges cannot

yet be explained.

19. J

Table 2.3. Summary of the results obtained in the test of the H-10 centrifuge.

-1Test system Maxlinum flowrtfti:, ml s (aqueous + organic)

Air turbine rotor (1MI0O rpm) Electric motor (21500 rpn)

KlCIO.Nysolvin/H,0

- " - / I «- " - /3 H NaClOj!- " - /5 M NaC10|[

n-hexane/H90- " - / I M HaCIOi

n MadOj

Benzene /H-0-" - / I M II NaCIO.-••- /3 H NaC107-" - /5 M NaClor

HIBK/H.O- " - / I M NaCI04-" - /3 M NaCIOZ- " - / 5 M NaCloJJ

282k2220

201510

19171512

19191919

0.3 M HDEHP in Shellsol-T/0.05 H HNO,-" - l\ M HNOj }

0.1 M Alamine-336 in Nysolvin/0.05 M HNO,0.1 H Alamfne-336 in Nysolvin/1 H HNOj0.05 M tetraphenyUrsoniumchloridein CHC1./0.I M HNOj

1.6

40

35

35

figure 2.5. Maximum flowrate through a H-10 centrifuge as a function of the centrifuge speed.

The outlets of the H-10 are equipped with pressure gauges for visual

reading and throttle valves for manual operation. Remotely controlled

valves were not considered necessary for two reasons: First ly , since

SISAK I was constructed we have changed to the gas je t target; so i t

is no longer necessary to have the f i rs t centrifuge in a "hot" area

requiring remote control. Secondly, the new centrifuge is more sensitive

and therefore requires a more direct contact between the operator and

20.

the valves. The new valve units have been designed to give a minimum

hold-up time.

Unlike the SISAK I , the four centrifuges of the SISAK 2 system are

situated in one compartment. This depends partly on the smaller dimen-

sions of the centrifuges, but the main reason is to obtain the shortest

possible liquid transport distance between the centrifuge stages (this

distance is only 10 cm of k mm tubing, corresponding to a hold-up time

of less than O.I s ) . The compartment is connected to the exhaust system

in order to remove radioactive and hazardous gases.

All SISAK 2 tubing is of PTFE and with 3 or k mm inner diameter.

The control panel has the same functions as the SISAK I control pa-

nel , but i t is bui l t in modules to allow a sîfflpîe replacement of failed

parts. I t is also equipped with a level alarm unit connected to resis-

tive gauges situated in the storage vessels (low revel alarms) and the

waste containers (high level alarms). These alarms have drastically

reduced the number of SISAK floods compared to the SISAK I system.

The SISAK 2 system wi l l be described more ün detail elsewhere [Ska 77b}.

In connection witn the description of lite wet chemistry equipment,

some words should also be devoted to the measuring cell» placed in

front of the detectors. In principle, the cells used have been of two

types, either cylindrical cells or coiled ce! Is . One of the cylindrical

cells used in this investigation has been a ~130 cnr titanium cell

suitable for measurements of nuclides in tfte h a l f - l i f e range 6 - 30 s.

The walls of this cell are I mm thick. In combination with th« self-

absorption in such a large liquid volume this makes the cell suitable

only for nuclides with few or no transi tiers below ~100 keV.

To allow determination of low-energy transitions, we used a MO cm

polypropylene cell with I mm thick vails and 10 mm to tat thickness of

the liquid volume. This cell allowed us to determine y transitions down

to ~20 keV. Another feature of this cell i * that i t Is possible to f i l l

i t with ion exchange resin or extraction cclumn material in order to

collect the desired activity in front of the detector.

A technique for measuring y-radiation with energies <20 keV and also

(}-radiât ion is presently under development {Bj8 7?] and wi l l be tested

in the near future.

21.

The coiled cells are used only in connection with measurements ac-

cording to the two detector delay (TDD) method. I I I , Aro 7 * ] . where

plug flow conditions are required. They simply consist of PTFE tubi g

<fi ; n n e r * 4 mm) wound around the Ge(Li)-detector head. When performing

TDD measurements, the inlet and outlet tubes of these cells must be en-

closed in an outer lead pipe, otherwise the activity in these tubes

w i l l affect the measurements in an unpredictable way.

2.4. Nuclear detection equipment

For a l l Y~ray and X-ray measurements performed in Oslo and Mainz

we have used Ge(Li)-detectors. The FWHM (at 1332 keV) of these detec-

tors has been 1.75 - 2.2 keV and the relative efficiencies 5 - 23 %.

The detectors used in Oslo were connected to a 16 K Nord-1 Computer,

while the nuclear detection equipment in Mainz included multi-channel

analyzers (16 K or U ) and standard electronics.

22.

3. SISAK CHEMISTRY

I t is possible to apply the SISAK technique to almost al l the e le-

ments of the periodic system i f proper chemical conditions are chosen.

Therefore, i t is of value to discuss typical SISAK configuration. Fig.

3 . 1 . shows a flow sheet of such a typical configuration involving four

mixer-centrifugal separator units. Roughly, this configuration consists

of five chemical separation steps, namely the transfer from a gas to

a liquid of the radionucl ides formed, decontamination from unwanted

radioactive species, extraction conditioning of the recycled organic

phase.

GAS-LIQUIDTRANSFER STEP

DECONTAMMATIsTEP

ION EXTRACTIONI STEP

BACKEXTRACTION 'CONDITIONINGISTEP JSTEP

figure 3.1. A typical SISAK configuration involving gas-liquid transfer of radioactive species,

decontamination, extraction, back-extraction and conditioning steps.

In the f i rs t step, the radioactive species carried by the clusters

(aerosol particles) in the jet gas are dissolved in a liquid phase.

This liquid might be either an aqueous or an organic solution. So far ,

we have only made use of aqueous solutions (0 - 1 M in HN0-). Some of

the problems related to the cluster dissolution w i l l be discussed be-

low.

The gas-liquid mixture is then fed into the degassing unit described

in section 2 . 3 - 1 - , where the j e t gas is swept off together with the

noble gases and, at least part ly, some other volati le compounds.

When working with radionucl ides formed in thermal neutron-induced235fission of U, the solution emerging from the mixer-degasser unit

contains a l l elements between Zn och Dy, except for the noble gases.

Therefore, i t is usually necessary to decontaminate the solution from

those unwanted elements which otherwise would be extracted in the third

23,

step together with the element under investigation. Preferably, the

D-va lue (the 0-va lue is defined as the ratio [S] Q r / [ S i , where [S]

is the concentration of a certain species) of the elements to be re-

moved should be >IOO, corresponding to an extraction yield of no re

than 99 %. The D-value of the element to be studied should be as low

as possible in this step, but D-values up to approximately O.I (cor-

responding to 10 % extraction) are acceptable.

In the third step, the element of interest is extracted into an

organic phase. The D-value in this extraction should preferably be

>IO (more than 90 % extracted), but D-values down to I (50 % extrac-

tion) are acceptable. The extraction of other elements should be as

low as possible. I f the extraction process in this step is extremely

selective, the previous decontamination step may be omitted.

The fourth step is a back-extraction of the required element from

the organic to an aqueous phase. The D-va lue in this step should be

low (preferably <0.5).

In the aqueous solution resulting from this step, there is s t i l l

one possibility of performing chemical separations, namely by choosing

a suitable type of product collector (ion exchanger, performed precipi-

tate or extraction column) in front of the detector. This opportunity

is usually ut i l ized when studying "long-lived" activit ies ( t . , , > 30 s) :i /it ***

when studying nuclides with half- l ives <30 s the measurements are nor-

mally carried out directly on the liquid flow leaving the back-extrac-

tion step. Nuclides with half- l ives <5 s are often studied by measure-

ments on the organic phase leaving the extraction step. In this case,

a somewhat reduced radiochemical purity of the fraction under investi-

gation has to be accepted.

Finally, the f i f t h step is a conditioning of the organic phase (which

is run in a closed circuit) to prevent accumulation of long-lived ra-

dioactive nuclides. The D-value of al l elements should be as low as

possible in this step.

3 .1 . General demands on the SISAK chemistry

There are some general requirements for the chemical systems to be

used in the SISAK chemical separation equipment. Some of these demands

are quite straight-forward, e.g. the solutions should not dissolve the

-^_^^b-^4

titanium metal in the centrifuges (this excludes the use of fluoride

solutions in concentrations >'O M and HCI in concentrations above

6-7 M). Other requirements are more specific to this technique. Thus an im-

portant factor affecting the chemical yields is the kinetics of the

solvent extraction process. The total contact time between the two

phases in each mixer-centrifugal separator step is of the order of

2 s in the SISAK I system, and less than 0.5 s in the SISAK 2 system.

Although solvent extraction is usually a rapid process, there are

systems where the extraction is not complete within these short periods

of time. Sometimes, i t is possible to speed up such chemical systems

by increasing the temperature or by making some changes of the chemical

composition. I f these measures are inadequate, the chemical system

has to be completely changed. As an example, i t can be mentioned that

i t was not possible to extract Ce(IV) from I M h SO^ solutions with

an acceptable y ie ld . In this case, the problem was solved by replacing

H^Ojj with HNOj.

Another problem when performing on-line chemical separations is

that acids are transported with the organic phase from one centrifuge

to another. This problem which is well-known in counter-current solvent

extraction processes was encountered already in the f i rs t SISAK experi-

ments described in ref. [1 ] . At that time, the effects of the acid

transport were decreased by install ing automatic t i trators in the

system. In the fission product experiments described in this thesis,

there were no disturbing acid transport effects since the aqueous

phases were usually not circulated but passed only once through the

system. I t i s , however, important to bear this effect in mind when

choosing the chemicals involved. Thus i t should be mentioned that an

attempt to use ascorbic acid as a reducing agent for Ce fai led comple-

tely because the acid was transported with the organic phase (cf. f i g .

3.8.) to the extraction step, where Ce(IV) was reduced and consequently

not extracted. This unwanted reduction could have been prevented by

running the organic phase in an open c i rcui t , but this would have been

a bad way to run the system from the waste point of view. Instead, we

rejected the use of ascorbic acid and developed another system (sec-

tion 3 .4 .2 . ) .

Another factor affecting the performance of the system is the vis-

cosity of the liquids passing through the centrifuges. A highly vis-

25. J

cous liquid necessitates a reduction of the speed of the centrifuge,

which in turn decreases the pumping capability. The viscosity problem

can usually be mastered by increasing the temperature of the solution.

This measure was sufficient for the most viscous aqueous phase so far

run, 6 M ZnCl.. The most viscous organic phase (2 H HDEHP in kerosene),

however, required a decrease of the centrifuge speed from the normal

ISOOO rpm to approximately 6000 rpm; these figures refer to the SISAK I

system. The new SISAK 2 equipment has not yet been run with this solu-

tion.

The considerations discussed above, concerning the general SISAK

configuration and the chemistry employed in SISAK experiments are

valid irrespective of the element studied. The rest of this chapter

w i l l , however, be devoted to a more detailed discussion of the chemi-

cal problems in connection with the separation procedures used for the

isolation of the elements La, Ce and Pr.

3.2. Transfer of radioactive species from the gas j e t system to a

liquid phase

The GJRT technique has now been uti l ized for almost a decade by an

increasing number of research groups but nevertheless only a few attempts

have been made to combine this fast transport method with a subsequent

mass or chemical separation. The infrequency of mass separation in com-

bination with a gas j e t system might be explained by the di f f icul t ies

related to the transfer of mass-separated species from the high-vacuum

inside the separator to the comparatively high pressure in the therma-

lization chamber of the gas j e t . Furthermore, in most cases, there has

been no need for a gas je t device after the separator because i t has

been possible to perform the nuclear measurements in a low background

area.

There are, however, some examples of gas j e t installations in con-

junction with mass separators e.g. the LOHENGRIN mass separator in

Grenoble [Mol 70, Mol 73, Arm 76]. Here, the pressure difference pro-

blems mentioned above have been less important because the ions passing

through the separator have sufficient kinetic energy to penetrate a

thin fo i l placed between the vacuum system and the thermalization

chamber. This high kinetic energy is obtained by ut i l iz ing the energy

given to the fragments in the fission process (most other existing

26. J

mass separators work with ions accelerated to 40 - 80 keV by a lowenergy heavy ion accelerator behind the ion source).

The f i r s t attempt to combine a GJRT system with chemistry was madeby Kosanke [Kos 73], who studied 38 min 63Zn formed in the decay of33 s •'Ga produced in a (p, 2n) reaction on Zn.

Kosanke simply mixed the je t gas containing benzene clusters carry-ing the radionuciides with a hydrochloric acid solution which thenpassed through an anion exchange column on which Ga and Zn were adsorbed,while Cu was continuously eluted. The y-ray spectra obtained by measure-ments on the column showed essentially no other act ivi t ies than thedesired 63Zn.

Similar experiments were performed by Puumalainen et a l . [Puu 73]»

who extracted Bi as the dithizone complex from a HC1 solution into a

CCI. solution and by Aumann and Weismann [Aum 7'tb], who dissolved the

clusters in 2 M HN0-.

The weak point of the above-mentioned methods of combining the GJRTtechnique with chemistry is that these systems are more or less dis-continuous, thus making no use of the continuity of the gas je t trans-port. When studying fission products or products from other nuclearreactions that yield a complex mixture of different elements, i t isnecessary to have a more selective (and consequently more complicated)multi-stage chemistry. Furthermore, i f the products are short-lived anon-line chemical technique may become compulsory.

Our f i r s t attempt to combine the SISAK on-line chemical separationsystem with the GJRT was performed at the Mainz Cockcroft-Wal ton acce-lerator in June 1974. In this run, the je t gas was mixed with a n i t r i cacid solution (at 25°C) of pH I.A in a stat ic mixer of the type men-tioned in section 2.3. The mixture was then fed into the SISAK-system,where the fission product La (cf. f i g . 3.16) was isolated in an un-expectedly pure fraction. Due to the low production rate (the targetamounted to 20 mg U and the 14 MeV neutron flux to < 5-10 n cm" s" ,

C lift ~

thus yielding ~ 2.5* 10" atoms La per second), these experimentswere regarded only as test runs. The f i r s t y-ray spectrum recorded bythe GJRT-SISAK technique is shown in f i g . 3.2.

The same approach to combine a gas j e t with chemistry was chosen

27.

2000

1000

!

0

1

j .

11 t

I l I £ :

M. .Il ? '-HM : ss "W L • i-

Ini ? h

1

e

I

nnl

g .

1 "I j

iJU..

***Ualii|la>a«acttwnE M I T >anfa 0-W00K.V1 >av pai t h i m lFiiat aavctrum racxtfatfwllh GJRT » H I M«1 lin« dMmtalry

•aw

u

» 5

J — 1

44.•

1

500

CHANNEL NUMBER

Figure 3.2. The first T-ray spectrum recorded after combining the GJRT technique with on-line

chemistry.

by Hellmuth and Val l i [Hel 75] who used jets of pure He at liquid N£

temperature, He + o i l , He + CCI. and Ar + o i l . These jets were mixed

with a blend of organic and aqueous solutions. The phases were then

separated in a simple centrifuge, and measurements were carried out

on the organic phase.

In order to get stronger samples, our next step was to install the

SISAK-system at the GJRT fac i l i ty in the Mainz TRI G A reactor. The

construction of this GJRT system has already been described in section

2.2.2. and refs. [V, Sil 77], and the discussion below wi l l therefore

deal only with the transfer to a liquid phase of the radionuclides

transported by the gas j e t .

The transported nuclides are attached to small ethylene or CO,

droplets ("clusters") dispersed in the inert carrier gas. The ethylene

je t was f i rs t used by Dautet et a l . [Dau 73] and Wiesehahn et a l . [Wie

73]* From investigations performed by Wiesehahn et a l . [Wie 74] we

know that the ethylene is present in the form of small droplets with

diameter <1 ym, provided that the ethylene gas pressure is above the

cri t ical point. The existence of these droplets has also been con-

firmed by Trautmann et a l . [Tra 75] by putting a glass fibre f i l t e r

into the je t stream. When doing so, a l l non-volatile activit ies leaving

the end of the je t capillary were stuck to the f i l t e r , which implies

that they are attached to particles.

28.

JL^^^.?-^^,?- .^^^*:^^^

The same phenomenon has been observed when using CO. clusters.

These clusters are obtained in an analogous manner to the ethylene

clusters , i .e. the pressure in the CO, container must always be kept

above the cr i t ica l pressure.

As carrier gas (for the ethylene clusters) we used N_ primarily

because of its low neutron activation cross section and low cost.

Furthermore, N, has a larger stopping power than e.g. He, so that the

inner dimensions of the thermalization chamber can be kept small, yielding

a short hold-up time (in the chamber used currently, the hold-up time

is "0.6 s ) .

When using CO- clusters, there is no need for an extra carrier gas,

since the CO, acts both as cluster material and carrier. This makes

the COj-jet extremely simple; there i s , e.g. no need for a gas mixing

chamber or equipment to keep the N,/C,H. flow ratio constant, necessary

when using the ethylene je t .

The chemistry inside the the rnral izat ion chamber is a f ie ld of inte-

rest that seem not to have been treated extensively.

The fission products are attached to the clusters both by adsorption

forces and by dissolution in the droplets. This is supported by the

dif f icul ty of completely dissolving the radioactive species in an

aqueous phase ( i f the activity was only adsorbed on the cluster surface,

they would be more easy to dissolve). We have observed that certain

elements, such as I and Br are much more firmly bound to the clusters,

probably due to chemical interactions with the ethylene molecules.

When using an inorganic j e t , l ike the CO, j e t , there seems to be no

such "hard-bound" effects, as expected.

In the very f i rs t gas j e t tests we obtained La in a pure fraction,

almost without any contaminants (except for the noble gas descendents).

When starting the gas je t experiments at the TRIGA reactor, we chose

the Ce chemistry as the test system. In this system (see further sec-

tion 3.4.) the main chemical separation is an extraction of tetravalent

Ce from 1 M HNO, into an organic phase. In this extraction step, the

equilibrium D-value for Ce is >50 (>98 % extraction), while the D-value

for La is 0.3-10 (0.3 % extraction), i .e. there is almost no La pre-

sent in the organic phase. However, large amounts of La were extracted

in this step when using the ethylene gas je t transportation system.

29.

The experiments were performed at room temperature (~22°C). Heating

the aqueous phase to 90°C before i t entered the gas-liquid mixer re-

sulted in almost no La extraction. We also checked the La extraction

at an intermediate temperature and found that i t lay between that at

22°C and that at 90°C.

The explanation of this phenomenon is probably that the ethylene

clusters are not totally destroyed in aqueous solutions at tempera-

tures below 70 - 80°C. Therefore, the undissolved clusters are trans-

ported with the liquid t i l l they meet the organic phase (mainly kero-

sene, i .e . a mixture of aliphatic hydrocarbons), where they are ex-

tracted due to their hydrophobic character. Normally, an ethylene

duster should be rapidly destroyed in water at room temperature, but

probably there are polymerization and cross-linking phenomena in the

thermal ization chamber, where the ethylene dusters are strongly i r ra-

diated with neutrons, y-rays and fission fragments, making the clusters

more d i f f icu l t to break up.Ihr

Other effects were that the amount of ?Ce passing the detector

cell decreased with increasing temperature, while the amount of

and Ce increased. The decreasing Ce yield is explained by the

extraction of La at low temperatures, which increases the amount of

Ce

grown-in

step

1*5,Ce formed between the Ce extraction step and the Ce stripping

For 'Ce and Ce this effect is negligible, since the primary

yield of La and La is comparatively low.

The increase in Ce and Ce might be explained by assuming that

the undissolved clusters extracted by the organic phase are not com-

pletely dissolved in that phase (e.g. because of cross-1 inking). I f so,

only the Ce from the dissolved parts of the clusters is stripped in the

next stage, while the rest of the Ce is transported with the undis-

solved clusters through that stage without being stripped. When in-

creasing the temperature, al l or nearly al l clusters are already de-

stroyed in the aqueous phase, and Ce is then extracted and stripped

without any interference from undissolved clusters.

To determine the efficiency of the transfer of radionuclides from

the gas j e t to the l iquid, we also performed a comparison between the

amount of Ce transported by the jet and the amount of Ce present at

the detector site [V]. The total Ce activity transported with the

clusters (calculated from the 317 KeV y-ray peak of Ce) was deter-

30. J

mined in a "direct catch" experiment, in which the gas j e t passed

through a glass fibre f i l t e r . As previously mentioned, a l l non-gaseous

products carried by the je t are trapped by this f i l t e r . In a second

experiment, we employed our ordinary chemical system for the isolation

of Ce (cf. f i g . 3.6.) and Its collection on an extraction column. Then,

provided that a l l chemical yields in the system were known, we were

able to compare the amount of Ce present on the column with the

amount that had been fed into the gas-liquid mixer (« the amount deter-

mined in the direct catch experiment). In this way, we calculated the

transfer of Ce from the je t gas to the liquid to be >80 %. When running

the gas-liquid mixing device under the right conditions, ( i . e . so that

the ethylene clusters are totally destroyed) this figure should also

be valid for most other non-volatile fission products. The missing 20 %

of the radioactive species is probably swept away in the degassing unit

or adsorbed in the gas-liquid mixer. This missing 20 % is probably some-

what overestimated, due to conservative calculations of the chemical

yields.

When using kerosene (boiling point ~li*0°C) as organic solvent there

are no severe problems connected with the high temperature of the aqueous

phase entering the f i rs t centrifuge. Naturally, contact with the aqueous

phase heats the incoming organic phase to 60 - 70 C, but this does not

effect the extraction yields significantly.

There are, however, chemical systems requiring more volati le or-

ganic solvents l ike chloroform (which is used in the chemical system

intended for the isolation of Tc). in such cases, i t is necessary either

to cool the aqueous phase before i t cones into contact with the organic

phase, or to use inorganic clusters that are readily soluble in water.

The only inorganic cluster so far tested with the Ce-chemistry is

C02> This gas je t system is simple, since C02 acts both as cluster

material and carrier gas. Furthermore, no disturbing polymerization

phenomena can take place between the CO, molecules.

As expected, the transfer of radionuclides from the C0_-jet to the

liquid was found to be almost independent of the temperature of the

aqueous solution. The yield increased slightly with the temperature,

but this effect was almost negligible. Thus, i t is no problem to run

a C02-jet at e.g. 30 - 40°C.

31,

rThe output from the COj-jet is about 50 - 60 % compared to the ethyl-

ene j e t . This indicates either that the C0_-clusters do not catch the

recoiling fission fragments inside the thermalization chamber as effec-

tive as the ethylene clusters, or that the C0_-clusters are less stable

(evaporate faster) than the ethylene clusters. Another possible explana-

tion is that the number of clusters present in the CO--jet is less than

in the C_H.-jet. The y?eld in the transfer of radionuclides from the

gas to the liquid is at least as high as for the ethylene j e t , i .e .

>80 * .

I t should be mentioned that we have also made an attempt to use the

o i l j e t developed by Wilhelm et a l . [Wil 74]. This je t works with small

droplets of olive o i l , which are added to the carrier gas (N_) in an

aerosol generator. This makes the o i l jet far more complicated to

handle than the ethylene and C02-jets. The test of the o i l j e t was not

successful, because the surface of the target was covered with an o i l

fi lm that prevented the fission fragments from recoiling into the

thermal ization chamber. This was probably caused by too large oi l drops

emitted from the aerosol generator.

3.3» The decontamination step

The choice of bis-2-ethylhexylorthophosphoric acid (HDEHP) as ex-

traction reagent for the lanthanides was based on the good selectivity

for this group of elements. HDEHP was f i rst employed by Peppard et a l .

iPep 57 a, b ] , who found that the separation factor between adjacent

lanthanides was greater than for other reagents; the average value

being 2.5. Since then, the extraction of lanthanides with HDEHP has

been studied by several authors (e.g. [Pie 63, Rao 65, Dub 67, Als 7k])

both for basic research and practical applications.

The extraction of various elements from HNO. solutions into HDEHP

is shown in f i g . 3.3. At acid concentrations £0.01 M, HDEHP provides

a good separation of the lanthanides, Y, Zr, Nb and Ho from mono- and

divalent elements like Rb, Cs, Sr and Ba. Elements like Zr, Nb and No

are so strongly extracted that they do not interfere in those cases

(La, Pr and long-lived Ce) when we measure the nuclides under investiga-

tion in an aqueous phase into which they have been back-extracted.

Nevertheless, there is often a need to look for the most short-lived

species by measurements made directly on the organic phase. Thus, in

32.

Figure 3 t3. Curves showing the extraction of mono-, d i - , t r i - and tetravalent metals from HNO, solutions(here represented by Cs, Sr, Ce and Zr) from UNO, solutions into 0.3 H HDEHP in an aliphatic solvent (nysolvin).

e.g. the Ce system where we extract Ce(IV) from 1 M HNO-, i t is necessary

to have a decontamination step where these elements are extracted, other-

wise the y-radiation from these nuclides, formed with high fission yields,

wi l l dominate the Y~ray spectra measured in the organic phase.

In the decontamination step used in the Ce and Pr chemistry systems

(the La fraction does not need any decontamination since La is always

measured in a stripped aqueous phase, cf. sections 3.6. and 3 -7 . ) ,

an aqueous phase consisting of 1 H HN0, is contacted with 2 M or 0.3 M

HDEHP in kerosene. 2 M HDEHP was used in the SISAK I system, but was

replaced with 0.3 M HDEHP when starting the experiments with the new

SISAK 2 equipment. The main reason for this change, which did not affect

the function of the decontamination step significantly, was to lower the

viscosity of the organic phase.

Fig. 3.^. shows extraction curves for some of the elements extracted

in the decontamination step. These curves, as well as the other extrac-

tion curves presented in this chapter, have not been determined "on-

l ine", but are a result of a series of test-tube experiments in which

the contact time between the two phases was 10 s. This contact time

yielded equilibrium D-va lues for a l l investigated elements except for

Nb.

As indicated by the curves, Zr, Mo and Y are extracted in a high

yield already at a HDEHP concentration of ~0.3 M. The light trivalent

lanthanides, here represented by C e ( l l l ) , are extracted only to 1 - 2 %.

33.

r%Ei1rKM

100

1

0 1

' •

Mo '

^ • "

/

, ,

vCa(IVI

.— m i •:

-. Nb

T«

-

Figure 3.4. The extraction of various elements from 1 H HNO, as a function of the HDEHPconcentration. Nysolvin (an aliphatic kerosene) was used as organic solvent.

Nb, which should be extracted as effectively as Zr and Mo, is extracted

only to ~25 %. This low yield depends on the slow extraction kinetics

in the Nb extraction, which is shown in fig. 3-5. Similar results for

the kinetics of the Nb extraction were obtained by Hagström and Svantesson

lHag 76] in a study of the extraction of fission products with HDEHP.

MOPh»M contact lirno.i

figure 3.5. Kinetics of the extraction of Nb from I H HNO, into 2 H HDEHP in kerosene.

The kinetic effects involved in the Nb extraction are disadvantage«»,

since they make i t d i f f i cu l t to extract a l l Nb in the decontamination

step. Consequently, the Nb remaining in the aqueous phase is part ia l ly

extracted in the second step in which Ce is e*.STacted.

Iodine is poorly extracted in this stage, both in the -1 state

(~2 %) and in higher oxidation states (<20 %); this means that I is

also present in the second extraction step, this I is oxidized at the

same time as Ce, and so ~20 % of this I w i l ! be extracted into the se-

cond organic phase. To minimize this contamination, tw have inserted an

I-decontamination step (an AgCI precipitation on a glass fibre f i l t e r )

prior to the second extraction stage (but feeftt** tht addition of any

oxidizing reagents). In order to keep I in the - I state, a small amount

of SO, (as Na2S0.) is added to the solution.

However, even i f the AgCl-column removes al l I present, the i pre-

cursor Te is always present in the liquid between the AgCI-column and

the second extraction step. Although the hold-up time is this liquid

volume is short (<1 s in the SISAK 2 system), i t is sufficient to givs136

a contamination of I in the second organic phase. Attempts to ex-

tract Te with e.g. dibutyldithiophosphoric acid have not been success-

ful (less than 20 % of the Te is extracted in test tube experiments).

IHNO31.M

Figure 3.6. The influence of the concentration of HNOj on the extraction of v»r!ous elements

Into 2 H H0EHP in kerosene.

35.

rFig. 3.6. shows the acid dependence in the 2 H HDEHP-HNO. system.

As can be seen, most contaminants are extracted in high yields from1 H HHOy while Ce(l l l ) is extracted to ~15 %. Fig. 3.7. shows a y -ray spectrum measured in the organic phase leaving the decontaminationstep.

sooCHANNEL NUMKK

Figure 3.7. Y"ray spectrum recorded in the organic phase leaving the decontamination step.

3.A. The Ce system

When describing the chemical systems used for the extraction of La,Ce and Pr, i t is suitable to begin with the Ce system, because i t wasthe f i r s t one studied. The other systems are, to a certain extent, basedon the Ce system.

Fig. 3.8. shows a flow sheet of the system used for the isolation ofthe fission product Ce. After the decontamination step previously des-cribed, where interferents l ike Zr, Nb and Y are removed from the aqueoussolutions, Ce(ll l) is oxidized to Ce(IV). In the second extraction step,Ce(lV) is extracted with 0.3 M HDEHP in kerosene, leaving the otherianthanide elements in the aqueous phase. The only other elements ob-served in this phase are, as mentioned in the previous section, Brand I grown-in between the AgCl-column and the extraction step (C2)and Nb.

In the third step (C3), Ce is back-extracted (stripped) as Ce (111)to an aqueous phase consisting of a reducing agent in 1 H HHO-. Whenstudying the most short-lived Ce-isotopes ( ' *9 ' 150Ce), the measure-

36.

IM HNO3 1M HNO3O« M H2SO40.2 M KjCrjO7

1M HNOj• • < M H

1 M HNOj01 M H2SO4 • • < M HJOJ0 « M KJCIZO; 0 « M HHjSOjH

OÎMHjC^O,

figure 3.8. Flow sheet showing the chemical system used for the isolation of Ce isotopes.

H - nixer, Dg » degassing unit, CI-C3 - mixer-centrifugal separator units, C » collector,

D • detectors, FP - fission products.

rents are carried out either directly on the organic phase (after C2)

or on the aqueous phase leaving C3. In measurements on the organic

phase, a slight contamination from Br, I and Nb has to be accepted.

In the aqueous phase measurements, the halogen contamination is sub-

stantial ly reduced but s t i l l present. However, the decay properties

of the short-lived fission halogens are quite well known, so this con-

tamination has caused no severe problems.

When studying the longer-lived Ce-isotopes, i .e . Ce, an ox i -

dation reagent is added to the reducing solution leaving Ce in order

to oxidize Ce( l l l ) to Ce(IV). The tetravalent Ce is then adsorbed on

an extraction column consisting of HDEHP on small plastic (e.g. PVC)

beads. This column material is enclosed in a cell placed in front of

the detectors. From this column, more than 90 % of the grown-in Pr is

eluted, which leads to very pure Ce Y"ray spectra. The Br and I iso-

topes, which pass through the cell within ~0.2 s can hardly be seen

in the spectra. Such a spectrum is shown in f i g . 7 . 1 .

The rest of this section wi l l deal with the main chemical steps in-

volved in the Ce separation, i .e . the oxidation of trivalent Ce, the

reducing str ip of Ce and the extraction column.

Ji41J[i_The_>gxiidation_and_extraçtion_of_Çe

The f i rs t attempts to oxidize Ce( l l l ) to Ce(IV) were performed with

0.1 M KBrO, in 10 M HNO, as the oxidizing solution. This method had been

employed e.g. by Peppard et a l . [Pep 57 c] in solvent extraction studies

of Ce ( IV) .

37.

rWe found that the bromate oxidation worked very well in test tube

runs, but in on-line runs, in which we tried to extract Ce(IV) from

10 H HNO, into 0.3 H HDEHP in kerosene and then back-extract i t with

a reducing solution that was I M in HNO,, we encountered severe problems,

because the HNO, in the oxidation stage was extracted by the HDEHP and

than transported with the organic phase to the stripping stage. This

effect made the system d i f f icu l t to run; within some minutes a l l the

HNO, circuits had the same acidity (in these experiments, the aqueous

phases were run in closed circui ts) . Furthermore, the bromate solution

had the disadvantage of being extremely corrosive and of requiring good

protection against generated bromine vapours.

After tests of other oxidizing agents, such as ozone, we f inal ly

chose the chroma te system. This is based on the method described by

Smith and Moore [Smi 56] , who extracted Ce(IV) from an aqueous solu-

tion containing HNO., H-SO^ and K.Cr-0^ into thenoyltrif luoroacetone

(HTTA). They chose 1 M HNOj, 0.5 H H^O^and 0.1 M K j C r ^ as the •

best composition of the solution. A n i t r ic acid concentration of I M

was also adopted by us, because at this acidity no other elements than

IHNO31.M

38.

Figure 3.9. The influence of the concentration of HH0, on th« extraction of various elements

into 0.3 H HDEHP in kerosene.

't

Ce are extracted to any significant extent (provided that Zr, Nb, Ho

and most of the halogens have been removed in preceeding decontamina-

tion steps). The extraction of some elements into 0.3 M HDEHP in kero-

sene as a function of the acid concentration is shown in f ig. 3.9- I t

is easily seen that the extraction of Ce(IV) has a minimum at ~0.2 H

HN0-. This is explained by an incomplete oxidation of Ce(lll) to Ce(IV)

at low acid concentrations. I f the solution contained only te t ravalent

Ce, the extraction yield curve should have been similar to the Zr curve.

I t is also seen that the extraction of I in i ts oxidized form (I . »

symbolizes the I present in a solution that is 1 M in HNO,, 0.1 M in

H.SOr and 0.05 M in K.Cr.O.) passes through a maximum. Therefore, i t

is important that the acidity of the n i t r i c acid solution does not

exceed 1 - 1.5 M. The extraction of Tc is indicated to provide com-

parison with an element that is not extracted at a l l (phase purity

monitor).

The effect of the K_Cr_07 concentration on the extraction is shown

in f i g . 3*10. I t is seen that Ce is already almost completely extracted

at low K-Cr.Oy concentrations. However, to be sure of a sufficient

oxidizing power in the solution (irrespective of possible reducing

agents present) we have used 0.05 M dichromate.

With respect to the normal potentials of the Ce(lIl)/Ce(IV) and

Cr(Vl)/Cr( l l l ) couples (1.M V and 1.33 V, respectively [Wea 68]), t r i -

valent Ce should not be oxidized by K-Cr-O- [Smi 56]. However, as indi-

cated in f i g . 3.11, which shows the extraction (« the oxidation) of Ce as

% Ealtaflad100 f—

Figure 3.10. The influence of the concentration of KjCrjO- on the oxidation of Ce. Since the

extraction yield of Ce(IV) is almost 100Ï, it is likely to believe that % extracted M Î oxidized.

39.

r% tmUÊOÊt

I«1 I

IC1.M

Figure 3.11. The influence of the concentration of Ce(IM) on the oxidation of Ce with 1 H HHOj,

0.1 H HjSO^ and 0.05 M KjCrjO-. As in fig. 3.10, the extraction yield was assumed to be approximately

equal to the Ce oxidation yield. The Ce concentration in the chemicals used in this experiment

was negligible.

a function of the Ce concentration, i t is possible to use dichromate as

oxidizing agent i f the Ce concentration is below ~10 M. In SISAK on-line

experiments, the Ce concentration is always below this value.

The oxidation of carrier-free Ce by dichromate is probably made possi-

ble by Che continuous removal of Ce(IV) into the organic phase, which

shifts the Ce(l l l ) ->• Ce(IV) equilibrium to the tetravalent side.

To stabil ize Ce(IV), the oxidizing solution contains H.SOr [Smi 56] .

In our experiments, we had to decrease the H-SO. concentration to 0.1 M

to minimize the kinetic effects caused by formation of Ce(IV)-sulphate

complexes.

To promote the Ce oxidation, the mixed solution (I M HN0, from the

and 0.2 M K2Cr20? fromdecontamination step and 1 M HN0-, 0.4 M Hj

a storage vessel) are kept at ~80°C. When running the SISAK 2 system at

ful l speed, the high temperature in combination with the short contact

times lowers the Ce extraction yield to approximately 60 %. We presume

that the decreased yield is due to kinetic effects in the Ce oxidation

process. This is supported by the fact that the yield increases when

increasing the contact time between the oxidizing solution and the

solution from Cl. However, an increased contact time also means an in-

creased hold-up time and so we usually accept the decreased y ie ld .

A disadvantageous effect observed when using K-Cr-Oj as oxidizing

agent is the extraction of Cr into the organic phase. This extraction

increases the oxidation potential of the organic solution so that Ce

40. J

becomes more and more d i f f i cu l t to back-extract with a reducing agent

in the subsequent stripping stage. The Cr extraction increases with

increasing HNO- concentration. I t is therefore important to choose a

HNO- concentration sufficiently low to prevent too much Cr from being

extracted and sufficiently high to keep the trivalent Ian than ides in

the aqueous phase. 1 N HNO- seems to f u l f i l l these requirements.

wooPIUM contact lim«, s

Figure 3.12. Kinetics of the extraction of Cr fro™ 1 HFigure 3.12. Kinetics of the extraction of Cr fro™ 1 H Ht» 0.1 M H.SO,, and 0.05 H K-Cr 0

into 0.3 M HOEHP in kerosene. The Ce tracer was added as K^'crjO .

The time-dependence of the Cr extraction is shown in f ig . 3.12. I t

is important to remember that the total volume of the organic phase is

«•5 1 , while the organic flow-rate is normally ~15 ml/s. This msans that

the organic phase volume passes through the centrifuge approximately

12-15 times/h. The contact time (mixer + centrifuge) is ~l s per circu-

lation (~«» s in the SISAK I system) which yields a total phase contact

time of ~15 s/h (60 s/h in the SISAK I system). From this figure i t is

easily understood that the organic phase has to be changed at least

once a day in order to prevent a disturbing Cr accumulation. One way to

aviod this would be to strip the Cr in a conditioning step. However,

the Cr complex extracted into the HDEHP seems to be extremely stable

and almost impossible to back-extract into an aqueous phase.

2i5i.21_The_reduçtiion_and_baçk;extraçtion_of_Ce

To back-extract tetravalent Ce from an HDEHP solution into an aque-

ous phase is d i f f icu l t . We therefore decided at an early stage that the

most suitable method would be to reduce Ce(IV) to Ce( l l l ) before back-

extraction. Table 3 -1 . shows the results of an investigation of possible

try of the results obtained in the test of reducing agents for Ce.

Str ip Solution

0.0S H NaN02 + 2 * » a In 1 H

0.0$ N HaN02 0.0$ N HjOj ir

O.I N ascorbic acid in 1 « HI

0.01 H N*NO2 in 1 H HNOj

0.05 N H202 + 0.0$ H HH2S0jH

0.05 H NaN02 + 0.02 M HJNNJH

1 H HCI

HH0?

1 1 M

in 1

in 1

I M ,

H HHOj

H HNOj

Ce back-extraction yie ld. *

9B9695878560

58

reducing agents. The most effective of these solutions, 0.1 M ascorbic

acid in 1 M HNO and 0.05 M NaN02+ 2 % lactic acid in I M HNOj, could

not be used on-l ine, because the ascorbic acid was extracted and the

NaN(>2 solution did not f u l f i l l the requirement to allow an easy re-

oxidation of Ce by adding dichromate before the extraction column.

Figure 3.13. Curve A shows the back-extraction yield of Ce(IV) from an organic pnase consistingof 0.3 M HBEHP in kerosene into 0.0 - 0.1 M NHjSOjHin 1 H HHOj. Curve B shows the yield whenback-extracting with 0.0 - 0.1 M H 20 2 in 1 H HNOj, while curve C shows the yield for equalconcentrations of NHJSOJH and »fa in 1 H HN0,.

Sulphamic acid, NH2S0,H, in 1 M HNO, proved to strip Ce to ~15 % in

a broad concentration range (:>0.03 M) as shown in f ig . 3.13. A test

of H202 in 1 M HNO, gave a similar curve (cf. the same f i g . ) . Up to

75 % of the Ce could be back-extracted with H.O.. Hrwever, to obtain

an even more effective stripping, the concentration of H.0, had to be

increased so much that Ce became d i f f icu l t to re-oxidize (this step

wi l l be discussed in the next section).

The third curve in f ig . 3-13 shows the stripping yield when using

equal concentrations of NH2S0,H and H20, in 1 M HNO,. In this case,

there appears to be a weak synergistic effect, so that the strippingyield at 0.05 M Nr SOjH + 0.05 M h"2(>2 (85 %) corresponds to the yieldat 0.18 H HJOJJ. Due to the re-oxidation demand mentioned above, theformer composition is to be preferred.

In on-line runs, the yield of the NH2S0-H - H202 s t r ip is as lowas 30 - 40 %, probably because of the chromium contents in the organicphase and the kinetics of the reduction step. A proportion of the halo-gens is also back-extracted in this step, while Zr, Nb and Ho remainentirely in the organic phase.

The extraction of Cr is a factor of great importance in the subse-quent Ce back-extraction step. Fig. 3-14 shows the back-extractionyield of Ce with the standard str ip solution (1 M HN0,, 0.05 M H-O,and 0.05 M NH2S0,H) as a function of the phase contact time betweenthe chroma te solution and the organic phase in the extraction step.The contact time in the back-extraction has been 10 s. The curveclearly indicates that Ce is much more d i f f i cu l t to back-extract i fthe organic phase contains Cr. The decreasing Ce back-extraction yieldis probably due to a destruction of the reducing agent by the hexa-valent Cr.

WOOtion and tha «raamc phaia.s

Figure 3.Hi. The back-extraction yield of Ce as a function of the contact time between the phases

in the previous extraction step. The contact time in the back-extraction step was 10 s. The

standard reducing solution (0.05 H HjOj , 0.05 H NHjSO^H and 1 M HNOj) was used.

The most short-lived Ce nuclides were measured directly in the re-ducing str ip solution described in the previous section or in the or-ganic phase after C2. In these measurements, the solution passes through

a detector cell with a mean hold-up time of I - 10 s. However, whenstudying ' 5~' 8Ce, CT,/2>45 s) i t is important to collect the ac t iv i -ty in front of the detector to obtain a good count rate. The simplestway to achieve this is to insert a cation exchanger in front of thedetector.

A disadvantage arising from the use of a cation exchanger column isthat is adsorbs not only Ce (to ~60 %) but also the grown-in Pr. There-fore, the Y-ray spectra measured on such a column w i l l soon be dominatedby peaks belonging to 2k min Pr and 2 min Pr.

In order to avoid this contamination from grown-in radionuclides,we used an extraction chromatography column on which Ce (as Ce(IV)) wasselectively absorbed. This technique was f i r s t used by Hoffman and Michel-sen [Hof 66] who used a column consisting of HDEHP on polytrifluorochloro-ethylene beads. When eluting this column with a strongly oxidizingsolution, Ce(IV) was retained on the column while the tr ivalent decayproducts Pr and Nd were continuously removed.

To fac i l i ta te the use of a HDEHP column, in the SISAK experimentsthe solution from the back-extraction stage must be made oxidizingagain. This is achieved by injecting a solution consisting of 1 M HN0»,0.4 M HJSOJ, and 0.2 M K2Cr20? directly after C3. To add the correctamount of this oxidizing solution is easy, because excess H20_ formsa blue complex with the chromium.

The column had to f u l f i l l at least four requirements. The most im-portant demand was that i t should catch at least 60 - 70 % of theCe(IV) passing through i t in a wide flowrate range (2 - 15 ml s" 1 ) .Another requirement was a low pressure drop in the column; otherwisedi f f icu l t ies would be encountered in making the thin-walled detectorcell withstand the pressure from inside. The column had also to beeasy to prepare and storable for at least some weeks.

Table 3.2. Summary of the results obtained in the test of extraction chromatography columns.

Column type Ce retention, %

HDEHP/PVC

HOEHP/Corvic

HDEHP/PTFE

HDEHP/polystyrent

99

86

78

"•9

Of the column materials tested (table 3 .2 . ) , PVC was found to be

superior because HDEHP is not only adsorbed on the surface of the PVC

beads, but also dissolved in the beads. This makes i t possible to use

the columns for a long time (up to 10 000 column volumes have been

pumped through) without washing the HDEHP away. At 10 ml/s f lowrate,

the HDEHP/PVC column absorbs —80 % of the Ce, provided that the d ia-

meter o f the PVC beads is 0.25 -0.50 mm. The other requirements men-

tioned above are also f u l f i l l e d by th is column.

An advantage of the HDEHP/PVC column is that Ce is so strongly

absorbed that we have been able to perform the Ce h a l f - l i f e determina-

t ions while e lu t ing the column wi th ox id iz ing so lu t ion . More than 90 %

of the grown-in Pr is eluted immediately. The remaining ~10 % of the Pr

is probably formed in the decay of Ce atoms which have been absorbed

into the beads. I t should also be mentioned that Ce ha l f - l i ves measured

with or without Pr e lu t ion are equal w i th in the error l i m i t s , indicat ing

that no Ce is e lu ted.

2 ifj;A i_Some_OBeratigna2_çharaçteris t^çs _of_the_Çe_sys tern

When running a SISAK experiment, the goal is to transport to the

detector s i t e as many atoms of the nuclide under invest igat ion as

possible per time un i t . The f rac t ion passing the detector ce l l should

also be as radiochemically pure as possible. The pur i ty is obtained

by employing a proper chemistry, as discussed above. The number of

atoms per time u n i t , however, depends on the flowrates and the delay

properties of the actual system conf igurat ion. Thus, when running

Ce and Ce (ha l f - l i ves 3 minutes and 14 minutes, respectively) a

delay tube is inserted before the extract ion step to allow 25 s La

and 8 s La to decay to Ce before the Ce separation. (The cumulative

f iss ion y i e l d of 11f5Ba + lZ|5La and lZ|6Ba + 1 i f6La is 3.3 % and 2.1 %

respectively [Wea 63] ) . The second organic phase and the reducing s t r i p

phase may be run qui te slowly because the ha l f - l i ves of these Ce iso-

topes are long compared to the transport times in the system.

When studying Ii>7Ce and 1l|8Ce (ha l f - l i ves 56 s and 48 s respect i -

ve l y ) , there is no need for an extra delay tube p r io r to the extract ion

step since the ha l f - l i ves of 7La and La are short (~2 s ) . Further-

more, the f iss ion y ie lds of l i f 7La and ' ^ L a are so small (0.9 % and 0.5 %

respectively [Wea 63]) that they do not markedly af fect the amount of

Ce present in C2. In the study of l<f7Ce and | i | 8Ce, a l l f low-rates have

rto be kept high to minimize the decay before the detector s i te . The

flowrate between C3 and the extraction column i s , however, rather low

because of the pressure drop in the column. This causes an extra delay

of some seconds, but since the half-l ives of Ce and Ce are quite

long compared to this delay, i t has no effect on the measurements. The

low flowrate of the reducing strip solution also has a favourable ef -

fect on the back-extraction yield in C3 (the contact time is increased).

The most short-lived Ce isotopes, °Ce and " c e (6 s and k s , re-

spectively) require the whole system to be run at maximum speed. To

increase the flowrate in the reducing strip solution, the extraction

column material in the detector cell is omitted and the measurements

are performed directly on the liquid phase. As there are kinetic effects

involved in the back-extraction, the higher flowrate lowers the back-

extraction yield about 40 %. Therefore, these nuclides were often mea-

sured (especially when performing coincidence measurements) in the

organic phase between C2 and C3, although in this case Nb is also pre-

sent.

3.5. The Pr system

There are two possible ways of achieving a chemical separation of

Pr isotopes for subsequent nuclear measurements. The f i r s t , which is

less selective, is to separate P r ( l l l ) from the other Ian than ides via

a multi-stage extraction process. However, the separation factor bet-

ween two adjacent lanthanides is <2.5 for al l known extraction rea-

gents, thus a decontamination factor of 10 would require 8 extraction

steps. As one step requires at least 1 s, this method would be slow

and i t would also require too much equipment to be used.

The second method, which has been uti l ized in this investigation,

is to make a selective separation of Ce(IV) from the rest of the

fission lanthanides and then milk the grown-in Pr from Ce. I t is

obvious that such a separation system wi l l be rather similar to the

system used for the isolation of Ce. Fig. 3.15.» which shows the sys-

tem used for the isolation of ' ^ " ' ^ P r (when studying 1 5 0Pr, the ion

exchange column is replaced with a tank detector c e l l ) , shows that the

decontamination step and the Ce extraction step are the same as in the

Ce system. The difference i s , that the reducing strip in C3 which was

designed to back-extract Ce (together with Pr) is changed to an oxidi-

r

1 M HNO3

Figure 3.15. Flow sheet showing the chemical separation system used for the isolation of Pr

isotopes. H - mixer, Dg » degassing unit, C1-C3 » mixer-centrifugal separator units, C * collector,

0 - detectors, FP • fission products.

zing str ip that keeps the tetravalent Ce in the organic phase, while

Pr is back-extracted. The back-extraction yield of Pr is high {>90 %)

and the Pr fraction is almost free from contaminants. A small amount

of Ce is a l l that can be measured on the ion exchange column; when

measuring Pr directly in the aqueous phase I and Br contamination

can be detected.

The retention of P r ( l l l ) in the cation exchange column (Dowex

50W x 8, 50 -100 mesh) seems to be high (>60 %) even at flowrates of

10 - 15 ml/s. These high flowrates are faci l i tated by the low pressure

drop in the cation exchange column.

The Pr isotopes studied in this investigations have A > 147, there-

fore there has been no need for any extra delay before the Ce extrac-

tion (as is discussed in section "i.k.k.). On the contrary, the aqueous

phase passing Cl and C2 has to be run as fast as possible to prevent

too much Ce from decaying before i t is extracted. This is especially149 150

important i f studying Pr or Pr, which are formed in the decayof 6 s U 9Ce and k s 15°Ce.

In the organic phase between C2 and C3, there must be an extra de-

lay to allow Ce to decay to Pr before the Pr back-extraction stage.

When studying Pr and , Pr, this delay was ~60 s, corresponding to

a l i t t l e more than one ha l f - l i f e of the Ce precursors. An even longer

delay would have been advantageous but i t would have caused some prac-

47.

t ical problems (e.g. too long tubing; 60 s corresponds to «60 m of k ran

tubing). Furthermore, the 'Pr and Pr activity was sufficient with

a 60 s delay.

In the Pr experiments we used a delay of ~15 s, which is moreihn

than two half- l ives of Ce. A longer delay would have had no great149influence on the Pr countrate.

The nuclides 1 i#7~ l l f9pr a r e a j j relatively long-lived (lA - 2 min)

nuclides with comparatively short-lived mothers (56 - 6 s ) . This makes

the choice of the delay intended to allow the growth of Pr fa i r ly straight

forward. For the most short-lived Pr nuclide studied, Pr, the situa-

tion is that the mother and daughter nuclides have almost the same half-

lives (4.0 s and 6.2 s , respectively). This necessitates an optimaliza-

tion of the growth time. Such an optimalization was done by finding

the maximum value of the function

Np r ( t ) - "CeXPr " ACe

(3-D

where NPr the amount of Pr present in the Pr back-extraction stage.

The most favourable delay time was found to be 7 s.

The flowrate of the aqueous phase is kept at a comparatively low

value (~10 ml/s) in the Pr experiments in order to obtain a

good Pr retention, while i t has to be run at maximum speed in the '

experiments because of the much shorter h a l f - l i f e of this nuclide.

Pr

3.6. The system

In the separation of La from fission products, i t is not possible

to make a selective isolation as for Pr (see section 3.5. )* Therefore,

i t is necessary to separate La together with the other fission lantha-

nides. In this mixture, the La, Ce and Pr activity dominate, because

of the rapid decrease of the fission yields in the mass region A >I5O.

Thus, the separation problem is mainly a problem of decreasing the Ce

and Pr activity present in the La fraction (to be correct, the La frac-

tion should be called the Ln( l l l ) fraction, but since La dominates the

spectra measured in this fraction, we prefer to call i t the La fraction).

We have tried to measure nuclides like ' 5 l Nd , '^2Nd, and 15^Pm in the

La fraction after collection on an cation exchanger but we have not

found any Y-ray energies belonging to these nuclides. Therefore, we

%^-V,£-a?^S>^:?>

have concluded that there are probably no lanthanide nuclides with A

>150 present in the La fraction. However, as discussed in [ V I I I ] , we

have found weak y-ray energies in the measurements on | /*'»" l ' l8La which

might belong to heavy, short-lived lanthanides like Nd and Pm.

1 M HNO301 M H2SO«0 05 M K2Cr2O7

1 M HNO3OOS M NH2SO3H0.05 M H2O2

Figure 3.16. Flow sheet showing the chemical separation system used for the isolation of l**"'*8La.

M - mixer, Dg - degassing unit, C1-C3 - mixer-centrifugal separator units, C - counting cell,

0 » detectors, FP - fission products.

In the chemical separation system used for La (cf. f ig . 3*16),

we extract al l t r i valent lanthanides present from a HNO solution of

pH ~1.4 into 0.3 M HDEHP in kerosene. The pH is measured on-line by a

standard pH-electrode (glass electrode f i l l e d with KC1) situated in a

special type of flow cell [And 69]. I t is determined after the extraction

step to obtain the true pH value in the extraction process. The value 1.4

refers to 70°C, which is the temperature in the extraction step (the

aqueous solution from the degassing unit i t at ~70°C when i t comes into

contact with the organic phase, the temperature of which is kept constant

by continuous cooling).

In this extraction step, almost 100 % of the lanthanides are extracted

into the organic phase (cf. f ig . 3 .9 . ) . Some other elements, like Zr, Nb

and Mo are also extracted, while the halogens, probably present mainly

as l" and Br", are extracted only to <0.5 %< Other elements are not extrac-

ted at this pH.

The kinetics in this extraction step is fast, i .e . the extraction yield

was not affected by the short phase contact times (compared to test tube

experiments) ut i l ized in the SISAK experiments.

In the next step, Ln( I I I ) is back-extracted to an oxidizing aqueous

phase consisting of 1 M HNO3> 0.1 M r SO^ and 0.05 M CryD-,. This back-

extraction is almost complete with respect to La and Pr, while more than

95 % of the Ce remains in the organic phase as Ce(IV). Host of the Ce

observed at the detector position has probably grown-in during the trans-

port from C2. The Pr passing the detector has presented no severe prob-

lem, because the T"ray energies of these nuclides were known from ex-

periments with the Pr system. Furthermore, almost no Pr or Pr is

present because these mass chains pass the La back-extraction stage

mainly as ]>llCe and l W Ce (the primary fission yields of 1If7Pr and 1l |8Pr

are low [Wea 63] ) . Pr can be observed since i t has a significant

direct fission yield and because 9Ce is more short-lived than 1 l f7 l> | l>8Ce.

Pr appears more strongly in this fraction than in the Pr fraction.

Some of the 5 Pr data (section 8 .5 .30 are therefore based on spectra

measured with the La chemistry.

Since La is both extracted and back-extracted in a high yield (a l -

most 100 %), i t has been no problem to obtain strong La sources. In

fact , although a cation exchange column in front of the detector wouldILL

have been favourable for La, we did not use such a column because

of the good sample strength.

50.

The system used for the isolation of ' * * " ' * \ a | s t n e easiest sys-

tem to run of those discussed, because of the absence of ion exchange

columns which cause pressure drops. No extra delay tubes are necessary

in this system. Some of the La precursors are long-lived compared to the

transport time from the degassing unit to the extraction step ( Ba -

11.9 s , 1/>5Ba » 5.6 s and 1I|6Ba = 2.2 s [See 74] ) , but the countrate144 146

of La and La has nevertheless been sufficient. Running the sys-146 144tern without such delays also increases the ratio La/ La, which

is favourable when studying La.

Keeping the pH in the extraction process at a constant value of 1.4

was also easy to achieve by on-line mixing of pure H,0 and 0.2 N HNO,

before the gas-liquid mixer unit. Once the correct pH was obtained

i t remained constant during runs of several hours.

3.7» The JLa system

To facilitate nuclear measurements, e.g. Y~Y~coincidence measure-

ments, onJLa (T1/2 14 min), it should be possible to use the

144-148La system and Insert a cation exchange column in front of the

detector. After a sample collection time of 20-30 min, one could then144-148.

allow La to decay for 5 min and then measure the mixture of4 4 4 4 6 .46,.47,.48pr

0 3 M HDCHP "i Iwraiene

^——It»»*,

rr

pH-7«

1

1 " C O

IMHNOj• 1 M H2SO4

I"«*

4Figure 3.17. Flow sheet showing the chemical separation systen used for the isolation of La.

M - mixer, Dg • degassing unit, C1-C3 - mixer-centrifugal separator units, C • collector,

D • detectors, FP - fission products.

However, this is a bad way to run an on-line separation system l ike

SISAK. To take advantage of the continuity of the SISAK technique, the

chemical separation shown in f i g . 3.17 was developed. Chemically, i t is)44-]48

identical with the La system, i .e . La is extracted into 0.3 M

HDEHP in kerosene from a sl ight ly acidic HN0- solution (pH ~1.4) and

back-extracted with an oxidizing aqueous phase (1 M HN0-, 1 M H.SO.

and 0.05 M K.Cr-O-J. La is then collected on a cation exchange column

(Dowex 50 W x 8, 50 - 100 mesh) which is renewed every 30 minute to143

decrease the amount of grown-in Ce. However, to increase the radio-

chemical purity of the La source, we inserted a decontamination

step (Cl) in which a l l t r i valent I an than ides present are extracted

from a pH ~1.4 solution into 0.3 M HDEHP in kerosene.142,

In this step (cf. table 3.3.), the mass chain 142 is present mainly as

Ba, which passes through this step without being extracted. Mass

chain 143 is mainly present as 20 s Ba which too is not extracted.143

There is also some La in the solution; this La is lost in the de-

contamination step. This i s , however, not a severe drawback of the144 144

system. Mass chain 144 is present mainly as La and Ce which are144extracted in CI; there is also a great deal of 5.6 s Ba, which i s ,

142 143as in the case of Ba and Ba, le f t in the aqueous phase. The masschains 145 and 146 arrive at Cl mainly as La and Ce which are extracted.

145In the aqueous phase leaving Cl there is only a l i t t l e 5.6 s Ba and

51. J

Table 3.3. Independent fission yields for the members of the A • 142-150 p-decay chains. The

yields are expressed in per cent of the total fission yield of each mass chain.

1*5 1*7 •*9 15»

Cl M O.7f) 25 d.7») I* (1.0.) . (0.6s> 0

»a H (11 »in) H (20i) ** (12s) 3« (5.6s) 21 (2.2s)

U 11 (S3 »in) 2* (I* «I«) 35 (*2s) ** (25s) *7 (lit

C» • 2 (33K) 7 (28*.))

•r 0 0 0

0 0 0 1

II (7) « (?) 0 0

«1 (2.2s) 29 (is) 1* (?) 1 (?)

15 (3 »in) 27 (I* «in) 35 (Ms) ** ft»±) ** tts) 21 (*s)

0 * 12* min) S (12 »in) 19 (2.2 «in) 3* (2.9 »in) *7 (is)

me»: The half- l ives of the nuciidcs are indicated in parenthesis. Underlined half- l ives are fro» the present investi-

gation, the others have been taken fro» ref. ISee 7*1 . The fission yield data is fro» ref. [tfea 63) .

even less 2.2 s Ba (the system is run so that the transport time

from the target to Cl is »«6 s, i .e . most Ba and ~50 % of the *Ba

has decayed to La before Cl) . Finally, the mass chains 147 and upwards

enter Cl only as Ln( I I I ) and are completely extracted in this step.

Thus, in the aqueous solution leaving Cl there are 11 min Ba,

20 s 11|3Ba, 12 s ' ^ B a , 5.6 s |It5Ba and a small amount of 2.2 a 1 %Ba

present. This solution then passes a delay tube, the hold-up time of

which is ~15 s. This delay allows ~2 % of the l i f 2Ba, 4l % of the 1<>3Ba,

58 % of the Ba, 84 % of the ' pBa and 99 % of the Ba present to

decay to the corresponding La nuclides, which are then extracted in C2.

The organic phase is then pumped through a delay tube with a hold-4 4142L La

142up time of ~200 s. In this delay tube, only a l i t t l e La and

decay to Ce, while 96 % of the '^La and more than 99 % of )I>5La and

La decay. Finally, in the back-extraction step (C3), La and \a

are stripped, while the mass chains 144-146 remain in the organic phase

as Ce. Thus, Y -ray measurements on the cation exchange column show14?spectra of almost isotopically pure La. The only "contamination"

142is the small amount of La formed in the delay between C2 and C3 and

a l i t t l e 5Ce and Ce that have not been oxidized in C3.

52.

1433iZiIi_Sonie_ogeratigna2_çharaçteristi>çs_of_the _La_sy.stem

143The La system provides a good example of the fact that i t is

also possible to use the SISAK technique to study quite long-lived

radionuclides. I t would also have been possible to study a nucleus

like 3Ce, formed in the decay of ]k\a, with a rather simple off-

line technique, but since the SISAK-system was available i t was natural

to apply i t to the problem.

143The main difference in the operation of the La separation system,

compared to the chemical systems described previously is that a l l flow-

rates have to be kept low, usually 3-4 ml/s. For this reason, we were

forced to run the centrifuges at a low speed, only 8-9000 rpm (thisits

speed refers to the SISAK I system; the JLa chemistry has never been

run with the SISAK 2 equipment). Running the system at such low values

caused no extra problems.

3.8. The conditioning step

Without exception, the organic phase ran in a closed circui t ; a con-

sequence of this operation mode is that long-lived nuclides of the

extracted elements are accumulated in the organic phase. I t is there-

fore desirable to remove as many of these as possible in a conditioning

step prior to the extraction step. Thus the organic phase leaving the

back-extraction step passes a delay tank (hold-up time 5 - 10 min) to

allow most of the activity to decay to Ce (La system), Pr and Nd (Ce,

Pr systems). Then the organic solution is brought into contact with

a solution consisting of 1 M HNO-, 0.1 M H,02 and some lactic acid

(the I an than ides form strong hydrophi l ie complexes with lactate ions).

The purified HDEHP solution is then pumped directly to the extraction

step.

In the La systems, this conditioning is important. I f no conditioning

were employed, the Pr grown-in from Ce after the step where La is back-

extracted would be transported via the extraction step back to the La

back-extraction step, where i t would be stripped together with La. In

this manner, the radiochemical purity of the La fraction would be se-

verely decreased.

The on-line conditioning of the organic phase is combined with a

periodic change of the solution. In this way, elements like Zr, Nb

and also inactive Cr are removed.

53.

k. OATA AQUISfTION METHODS

A. I . Collection of y-ray singles spectra

Most Y-ray singles measurements have been carried out using the

detector cells described in section 2.3.2. For measurements on nucli-

des with half- l ives >30 s, we have used either the titanium tank c e l l ,

which yields a mean hold-up time of 10 - 20 s in front of the detector,

or the thin-walled polypropylene cell f i l l e d with ion exchange resin

or HDEHP/PVC beads which retain the element under investigation for

an almost unlimited time. Usually, the plastic cell was used due to

the low absorption of low-energetic y-rays in this ce l l . Nuclides with

half- l ives £30 s have been measured in the plastic cell without any

collecting material. The hold-up times in this cell have then been

between 0.5 and 5 s. These short hold-up times were used to reduce the

influence from long-lived nuclides.

In the y-ray measurements, the cell was placed at such a distance

from the detector that the dead-time did not exceed 5 %. Thus we have

avoided count rate shifts and other unwanted effects. The gain setting

was usually at 0.5 or 1.0 keV/channel to obtain a good energy resolu-

tion. For energy calibration purposes, we used standard nuclides like5 6 Co, 6°Co, 1 3 7Cs, 160Tb and l68mHo covering the range 50 - 2000

keV. The measured spectrum length was normally 2 K or k K channels. To

obtain good stat ist ics, we have measured spectra so that the integral

sum of the spectrum was &2>10 counts.

The evaluation of the Y~ray spectra obtained, as well as the half-

l i f e and coincidence data, w i l l be discussed in section 5.1 - 5.3.

4.2. Ha l f - l i fe determinations

The normal way to determine hal f - l ives, i f not by measuring ß-radia-

tion, is to collect the nuclide for a certain time and then measure

consecutive y-ray spectra during the decay. This cycle is then repeated

several times and corresponding spectra are added. This method, which

we call "the conventional technique" is easy to perform and i t gives

accurate data, though count rate shifts may occur at high sample strengths.

I t does not, however, take advantage of the continuity of the SISAK

5*.

technique; on the contrary, most of the activity is sent directly to

the waste container as no activity is allowed to pass the detector

cell during the decay measurement. Nevertheless, we have employed this

method both for measurements of half- l ives >30 s and also for the most

short-lived nuclides like '*7La and '*9Ce.

When measuring half- l ives according to the conventional technique,

we have had the problem that only 16 K core memory has been available

for spectrum storage, i .e . i t has only been possible to measure 16 x 1 K

or 8 x 2 K channels spectra etc. When measuring half- l ives for nuclides

with high y ray energies, we have therefore had to decrease the number

of points on the decay curves or, preferably, to use the TDD method

(cf. section 4 . 2 . 2 . ) .

In the h a l f - l i f e determinations, the number of cycles was chosen

so that sufficiently good counting statistics were obtained in the

last spectrum in the measuring sequence.

fji2i2i_Measurements_aççordî-ng_to_the_TDD_teçhni-gue

To take advantage of the continuity of the SISAK technique even

when performing h a l f - l i f e determinations, we developed the TDD (Two-

Detector Delay) method. This technique has been carefully described

in refs. [ I ] and [Aro lh] and therefore only the main principles wi l l

be mentioned here.

A TDD measurement requires two Ge(Li)-detectors equipped with coil

cel ls , i .e . teflon tubing wound around each detector head. To simplify

the subsequent data evaluation,these coil cells should be identical

with respect to volume and geometrical configuration. They should also

be identically situated relative to the detectors to obtain a reprodu-

cible geometry during the h a l f - l i f e determination.

The dotector cells are connected to each other via a delay tube of

known, variable volume. The inlets and outlets of the detector cells

must be shielded with an outer tube of lead to prevent the tubing of

one detector cell from disturbing the other detector. The delay tube

i tse l f has also to be carefully shielded for the same reason.

When evaluating TDD data, the ratio between the areas of the corre-

sponding y ray peaks in detectors 2 and 1 , respectively, are plotted

against the delay time, which is defined as the sum of the hold-up

55.

rtimes in detector cell 1 and the delay tube between the detectors. This

simple definition of the delay time is valid only i f the detector cells

are identical.

In a TDD measurement, i t is recommended that the dead-tine of the

detectors is kept as low as possible, preferably below 1 - 2 %. I f

there is a considerable dead-time, this w i l l affect the activity rat io

R9/R1 because the dead-time in detector 2 wi l l decrease when the delay

time increases. However, i f the dead-time of the detectors is known

for a l l delay times, i t is possible to correct the count rates when

performing the evaluation.

A pre-requisite for the TDD method is that plug flow conditions exist

in the detector cell tubing and the delay tube, i .e . the Reynolds' num-

ber Re must be >2300. To obtain Re >2300 is normally no problem with

the narrow tubes and high flowrates used in the SISAK experiments.

However, teflon tubing is extremely smooth and i t is therefore not

absolutely certain that there is a fully-developed plug flow pattern

during the TDD measurements. The contribution from laminar flow is

probably small, since the half- l ives obtained in TDD and conventional

determinations have been equal within the error l imits.

In the TDD measurements, 4 K spectra have been measured to f a c i l i -

tate h a l f - l i f e determinations of high-energy f r a y s . The number of

points on the decay curves have been 8 - 2 0 .

The TDD method has been employed for half- l ives within the range

5 - 50 s. For longer half- l ives the TDD method is less suitable be-

cause the length of delay tube necessary becomes too great. The

increased length causes pressure drop problems; 60 s delay which

requires M>0 m of tubing seems to be a practical upper l imit . The

lower limit of h a l f - l i f e accessible with the TDD technique depends on

the hold-up time in detector cell 1. In these experiments, no half- l ives

shorter than 5 s have been measured with the TDD method.

fr-3. Y~Y coincidence measurements

The Y~Y coincidence measurements were a l l performed with the thin-

walled polypropylene cell with or without collector material f i l l i n g .

The detectors were placed with their axes 180° apart, i .e. they faced

each other. As wi l l be discussed in section 5 .3 - , this arrangement

caused some comptai scattering problems. These problems could have been

56. J

avoided by placing the detectors perpendicular to each other with a

lead shield inserted between them. This arrangement was not used be-

cause i t would have decreased the coincidence count rate too much.

The detectors employed were choosen so that one,of them had a high

relative efficiency to increase the coincidence countrate, while the

other had a good energy resolution (usually 1.75 keV FWHM at 1332 keV)

to fac i l i ta te an accurate calculation of the y-ray energies and to

fac i l i ta te multiplet resolution. Since the samples were often strong,

the detectors were placed up to 70 mm from the detector ce l l . To keep

the influence of random coincidences at a low level, the coincidence

countrate was not al lowed to exceed 20 events/s in any measurement.

The resolving time of the coincidence equipment was 20 - 25 ns.

This value is low enough to decrease the number of false coincidences

and high enough to allow y rays from not too long-lived met astable

states to be eff iciently detected.

As a general rule, we considered 10 coincidence events as sufficient

in a Y"T coincidence measurement. For Y"intense nuclides like the short-

lived La isotopes, there were no problems in obtaining 10 events, while1 3

other nuclides (l ike La and the Ce nuclides) were so y-weak that we

had to stop the measurements ear l ier . In spite of th is , acceptable

counting statistics were obtained.

57.

5. OATA EVALUATION

Though much data has been evaluated independently by the SISAK sub-

groups in Göteborg, Oslo and Mainz respectively, this description w i l l

be confined to the data evaluation methods used by the group in G5te-

borg.

In Göteborg, the hardware accessible for the SISAK Collaboration

consists of two computer systems, an IBM 360/65 system at the Göte-

borg Computing Centre (abb. GD) and a Honeywell H-3I6 system (16 K

core memory + 1.5 M dual disc memory; abb. H-316) at the Department

of Nuclear Chemistry, Chalmers University of Technology. Al l evalua-

tions requiring large memory part i t ions (I ike Y-spectra analysis, coin-

cidence evaluations etc.) have been made at the GD, while the H-316

system has been used e.g. for the p lot t ing of spectra and decay curves.

Before the description of the data analysis methods, i t should also

be mentioned that a l l data obtained from the Mainz experiments were

or ig ina l ly wri t ten on 7"track magnetic tapes. Before this data could

be handled at the GD, the tapes had to be sent to the Stockholm Com-

puting Centre, where they were converted to 9-track tapes (standard

in the IBM 360/65 system). These 9-track tapes have then been treated

at the GD either by the computer program SINGCONV [Aro 75] (Y~ray

singles spectra) or C0INC0NV [Aro 75] (Y~Y coincidence data). These

programs remove possible erroneous data records and convert the data

into a more compact form. All evaluation is then performed on the con-

verted data f i l e s . The data from measurements in Oslo were punched on

paper tapes and could thus be treated d i rect ly .

5 .1 . Evaluation of Y~ray singles spectra

To fac i l i t a te h a l f - l i f e determinations and calculation of relat ive

Y-ray in tensi t ies, i t is necessary to u t i l i ze a computer program that

performs not only a good computation of the Y~ray peak energy, but also

a proper calculation of the area of the peak. Most programs give good

energy values since i t is rather easy to f ind the maximum of a peak.

However, most programs available have or ig ina l ly not been tested on

the complex mixture of single peaks, doublets and t r ip le ts that forms

a Y-ray spectrum in the transit ion region between spherical and de-

formed nuclei , but only on a spectrum of a nuclide with a few well

spaced peaks with good s ta t i s t i ca l significance. Under these circum-

stances, most programs perform good peak area computations. The coun-

58. J

ting statistics in a real spectrum are often poor (e.g. the last point

in a decay measurements), at least in the high-energy part of the spec-

trum. In the La, Ce, Pr-region there are also several peaks squeezed

together between 90 and 400 keV. Many of these consist of more than one

component, which presents the program severe problems when calculating

the background under the peaks.

Our demand for a good program to evaluate r~ray spectra was that i t

should be possible to employ i t on complex spectra with poor as well as

good counting stat ist ics. The program had to be fast, since we had only

a limited computer time available. We therefore tested four programs

in Göteborg:GAMANL [Gun 67] , SAMPO [Rou 69] , GAMMAN [Nym 73] and AES 11

[Aro 76].

SAMPO showed good performance also when applied to complex spectra

and spectra with poor stat ist ics. I t is , however, slow and has therefore

been used only on extremely complex spectra. GAMANL is fast but is not

good for multiplet resolution. GAMMAN proved to be inferior to the three

other programs because i t finds far too many peaks. Furthermore, i t does

not calculate the peak areas but only the intensity relative to the

strongest peak in the spectrum, which makes i t impossible to use for

h a l f - l i f e determinations. AES 11, which is a modified form of the pro-

gram AES 1 [Lil 73] , seems to be the most suitable program for our app-

l ication. I t is faster than GAMANL and SAMPO and i t also gives reliable

peak areas.

To eliminate systematic errors due to the program used for the y-

spectrum analysis, most singles spectra have been evaluated by two

programs, usually AES 11 and GAMANL.

5.2. Treatment of h a l f - l i f e data

The determination of half- l ives after a SISAK experiment is based

on the peak areas calculated by the AES 11 program. Further evaluation

proceeds along two different paths, depending on the method employed

for the ha l f - l i f e measurement. I f the conventional method of sampling

and subsequent decay measurement (by measuring consequetive Y~ray

spectra) has been employed, we use the program DECSORT [Aro 76] which

follows the different peaks through al l the spectra (the number of

which is 8 - 16) of a decay measurement and calculates the countrate

and the proper time for each measuring point. In the other case, i . e .

59-

i f TDD measurements [ I , Aro 74] have been performed, we use a program

called T00S0RT [Aro 76] which starts by forming the ratio R2/R| o f t n e

countrates in detectors 2 and 1 , respectively. This output is then

treated by TDCSORT [Aro 76] , a program working analogously to DECSORT.

With the treatment by DECSORT or TDCSORT, the data has been conver-

ted to a l i s t of times and corresponding countrates (or TDD rat ios) .

From this l is t i t is easy to plot and evaluate the decay curves graphi-

cally. However, a SISAK experiment period often given 3 - 400 decay

curves and i t is therefore necessary to combine the graphical evalua-

tion with automatic h a l f - l i f e calculations. For this purpose, we have

so far employed the programs EXPALS [Gar 65] , FRANTIC [Rog 62] and

MINUIT [Jam 71], a l l working by means of least squares approximations.

EXPALS and FRANTIC were abandoned at an early stage, because they showed

numerical instabil ity when applied to complex decay curves with less

than 10 - 15 points. MINUIT, which was used for the calculation of many

half- l ives presented in this work, shows excellent properties in the

resolution of decay curves with up to k components. I t also works well

when there are few points on the decay curve or the statistics of some

points are bad, and i t is also easy to apply to curves showing growth-

and-decay (cf. paper [ I X ] ) .

5.3. Evaluation of T T coincidence data

The Y~Y coincidence data, which is stored on magnetic tapes as single

events (in "blocks" consisting of 102*» events) is evaluated by means of

the procedure COINAES [Aro 76]. This consists of two programs, CO I NAN A

[Aro 7k], which provides gate setting (either x or y-gates) and arithme-

t ic processing of the coincidence spectra (subtraction of compton correc-

tion gates etc.) and AES 11, which performs peak search and calculation

of peak areas. All coincidence spectra were plotted on the H-316 system

since the human brain often is better than a computer in deciding whether

a small peak in a coincidence spectrum should be regarded as a coinci-

dence or not.

A coincidence evaluation is begun by running an x- and y-projection;

i .e. projections of the coincidence matrix on the axes. In this projec-

t ion, which is both plotted and l is ted, gates are set around al l signi-

ficant peaks. A correction gate is set immediately below every main

gate. Usually the main gate is set according to the principle that a

60. J

S;-

narrow gate is better than a wide one.

In the complex projection spectra obtained in this mass region,

there are often double or even tr iple peaks. Such multiplets are gated

"channel by channel", whereafter the peak areas in the spectra obtained

are plotted as a function of the channel number of the gate. In this

way, i t is possible to decide which peaks are coincident with the dif -

ferent members of the multiplet.

The evaluation of coincidence data has also presented other problems.

As mentioned in section 4 . 3 . , the y-y coincidence measurements were

performed with the detectors facing each other (180° angle between them)

to obtain a good countrate. This was not the most convenient method,

because we encountered the problem of y-rays being compton scattered

from one detector to the other. This means that the energy of the comp-

ton electron, detected in one of the detectors, is registered in coin-

cidence with the compton y-ray, provided that this y-ray is detected in

the other detector. I f the detectors are placed far from each other

(more than ~50 mm apart) this problem is negligible due to the reduced

probability that the y-ray hits the detector (the solid angle is small).144-146Thus, in the La measurements, where we had to put the detectors

up to 140 mm from each other because of the strong sources obtained,

this scattering phenomenon caused no problems. In the measurements on

La, Ce and Pr, however, we had to bring the detectors closer to in-

crease the coincidence countrate. This measure introduced scattering

phenomena in the results.

In the evaluation, these phenomena appear as a coincidence between

two y-rays, the sum of which is the energy of a strong y-ray peak. I f

choosing the 724 keV y-ray originating in the decay of Ce as an

example, there wi l l be two coincident peaks, one at ~188 keV and one

at ~536 keV. 536 keV corresponds to the energy cf the Compton edge and

188 keV is the energy of the Compton scattered photon. In this example,

the scattering phenomenon caused no prob I ens; there is no Ce y-ray

peak at 536 keV so this coincidence was. immadiateiy disqualified.

In one case we had problems with the scattered compton y-ray, namelyIL 7-149

when measuring on Pr. In this measurement, the strongest y-ray

peak present is the 302 keV y-ray from the decay of T>r. The compton

edge energy of this y- l ine is ~I65 keV and the energy of the scattered149y-ray i s , accordingly, ~138 keV. Pr, which is measured simultaneously

61,

has Y"rays at both 138 keV and 1C>5 keV. Thus i t was d i f f icu l t to decide

whether this coincidence was only a scattering effect or a mixture of a

scattering effect and a true coincidence. There i s , however, a way out

of this problem. When gating channel by channel over a "scattering peak",

the corresponding peak in the coincidence spectrum moves one channel per

spectrum, because the sum of the two energies has to be constant. I f the

coincidence is a "real" coincidence, the peak in the coincidence spec-

trum does not move. The peaks caused by the compton scattering also show

a greater FWM-value than normal T-ray peaks.

The only y-rays causing problems like those mentioned above were the

724 keV T-ray from 11|5Ce, 218 and 317 keV from ' ^ C e , 269 keV from7 Ce and 302 keV from 8Pr. To exclude any influence of the compton

scattering effect on the decay schemes presented in sections 6 - 8 ,

the sums of a l l coincidences obtained have been compared to the r~rays

present in the singles spectra.

Another phenomenon is that when a strong peak is in coincidence with

a weak one, the possibility of a cross checking of the coincidence is

sometimes reduced. Thus, when gating on the weak member of the cascade,

the strong peak is easily seem in the coincidence spectrum, while i f

gating on the strong peak, the weak one "is drowned" in the spectrum

background, especially i f the strong Y~ray peak has many other coinci-

dences (like the strong 2+ -»• 0+Y-ray transitions in l W C e , ' ^ C e , l i |8Nd

and Nd). This phenomenon has not been a severe problem, though these

coincidences have had to be more cr i t ica l ly examined before being f i t ted

into a decay scheme.

The coincidence peaks have been classified into one of three groups:

strong (s) coincidences, the height of which are more than 3a (standard

deviations) of the background, medium (m) coincidences being 1 - 3a and

weak (w) coincidences, which have been less than la . Generally, strong

and medium coincidences have been regarded as unambiguous, especially

i f they have allowed cross-checking, while weak coincidences have been

handled in a more conservative way. Except for the transitions formed

in the decay of \a and 5La, there have only been a few weak coin-

cidences in the data.

5.4. Calculations of relative Y-ray intensities

The present investigation includes no p-ray measurements and we have

62.

therefore not been able to calculate any absolute y ray intensities.

I t should be possible to calculate absolute r~ray intensities for those

nuclides of which the absolute Y-ray intensities of the daughter are

known. However, we considered the errors in this method to be large, so

we wi l l not perform any absolute Y-ray intensity determinations before

we have the possibility to measure &-rays.

Relative Y-ray intensities are not so interesting from the nuclear

physics point of view but they do at least provide some information

about the probability of different transitions.

To fac i l i ta te calculations of relative y-ray intensities, a l l de-

tectors used were efficiency calibrated in the energy range kO keV -

4 MeV. As standards, we used aqueous solutions of nuclides like 56CoI 82and Ta placed in the detector cel ls , or these nuclides adsorbed on

the ion exchange resin contained in the detector cell employed.

J

r6. DATA OBTAINED FOR THE DECAY OF NEUTRON-RICH La ISOTOPES

When the present investigation of the light lanthanide elements was

commenced, the knowledge about the nuclear decay properties of neutron-142

rich La isotopes was, to a large extent, incomplete. For 84 min La,

most decay properties ( i . e . Y~ray energies, decay scheme etc.) were

well established, while for 14 min La only a few y - ray energies and

some levels obtained from nuclear reaction experiments on the stable

nucleus ] / |2Ce were known [Fri 6 1 , Ohy 71, Ehr 72, Les 72]. The data144

available for 42 s La were confined to the h a l f - l i f e and a few approxi-

mate Y~ray energies [Ama 67, Ohy 72a,b]. In the A » 145 mass chain, a

h a l f - l i f e of ~25 s had been assigned to a component that was assumed146

to be La [Gra 70]. For 8.5 s La only some y-rays obtained in mea-

surements on fission fragments were known [Che 7 ' ] , while 2.2 s La

and I s La were completely unknown.

The investigations performed with the SISAK equipment have yielded

knowledge about ha l f - l ives , y-ray energies and intensities for Laand also partial decay schemes for La. Furthermore, we have per-formed the f i rs t chemical separation of the nuclides La. Theresults obtained b, <- have been published in the papers [ I , IV, V I ,

V I I I ] .

This section w i l l be devoted to a thorough discussion of published

and unpublished SISAK data on the neutron-rich La isotopes. The SISAK

data wi l l also be cr i t ica l ly compared to other data available.

6 . 1 . Data obtained for the decay of \ a

The lightest La isotope studied by us is 14 min ]t*\a. l l | 2La,

( T i / 2minutes) was considered to be less interestinq not only be-

cause its decay properties were well known, but also because i ts long

h a l f - l i f e made i t less suitable for on-line experiments.143

Though JLa has a h a l f - l i f e that makes i t well suited for separa-

tion techniques such as precipitation, ion exchange and of f - l ine solvent

extraction, i t had been surprisingly l i t t l e studied. The high fission

yield of the precursor "Ts makes i t also easily accessible for on-

line mass separation, but i t had not been extensively studied by this

technique either. Therefore, we considered i t well worth studying, even

though the investigation required development of a special chemical se-

paration system (cf. section 3 .7 - ) .

64. J

The nuclide *La was f i rs t reported by Gest and Edwards [Ges 5 U ,

who determined its h a l f - l i f e as 19 min from the growth of Ce. A more

accurate h a l f - l i f e , 14.0 ± 0.1 min, was measured by Fritze et a l . [Fri 61 ] ,

who measured the (3-de cay from a chemically separated La fraction. This

group also determined a (L-value, 3*3 MeV, as well as many Y-ray energies.

In fact, the (L-va lue of Fritze et a l . is s t i l l the only one available.

The Y-ray energies were measured with a Nal (Tl)-detector. The results

published by Fritze et a l . did not include any decay scheme, because

only Y-ray singles spectra were measured.

The next group reporting data on La was Ohyoshi et a l . [Ohy 71] ,

who determined the energy of the two strongest Y~rays in a La fraction

chemically separated by an el ect romi g rat ion technique.

Ehrenburg and Aniel [Ehr 72] then performed a h a l f - l i f e determination

by measuring ß-rays from a mass-separated sample. The value obtained,

14.32 ± 0.73 min, was in agreement with the h a l f - l i f e measured by Fritze

et a ) . [Fri 61].

Buchtela [Buc 731 determined the h a l f - l i f e and some Y~ray energies

of La (with a Nal-detector) by measuring a La fraction chemically se-

parated by an electrophoresis method.

The latest measurement was performed by Blachot et a l . [Bla 76]. This143group separated La with the mass separators OSIRIS [Bor 71] and ISERE

[Bou 68]. The results include some Y~ray energies and a simple decay

scheme.

Some authors have contributed to knowledge of the excited states is

Ce by nuclear reaction experiments. Thus Groshev et a l . [Gro 70]

measured prompt Y~rays from (n,Y)~reactions on Ce, while Lessard et a l .142 143

[Les 72] uti l ized the Ce (d,p) Ce reaction. The lat ter group was

also able to determine the spin and parity for many of the levels by

performing angular correlation measurements on the scattered protons.143The spin of the ground state of Ce had earl ier been determined to

be 3/2~ by Naleh [Mal 65] who ut i l ized the atomic beam method.

A central question arising when studying radioactive nuclides by

means of a chemical separation technique is: What is the mass number

of the nuclide studied? To be able to consider the mass assignment of

65.

ra nuclide as unambiguous, we have used three cr i ter ia of which at least

one must be f u l f i l l e d . These cr i ter ia are:

1) The actual h a l f - l i f e and Y-rays have been observed in experiments

with a mass-separator known to have a good mass resolution.

2) The h a l f - l i f e of the nuclide corresponds to the h a l f - l i f e obtained

from the growth of a daughter nuclide with a well defined mass number.

3) The Y-rays showing a certain h a l f - l i f e can be f i t ted into a level

scheme obtained in a nuclear reaction experiment on a stable nucleus.

When regarding La, the cr i ter ia 1) and 3) are f u l f i l l e d . As men-

tioned above, Ehrenburg and Ami el [Ehr 72] observed the 14 minute act i -

vity in the mass chain A * 143. Furthermore, the excellent agreement143between the level scheme obtained by us from the ß -decay of La and

the levels obtained by Lessard et a l . [Les 72] also makes the assign-143ment of the 14 minute activity to La unambiguous.

The h a l f - l i f e of La was determined by the conventional technique

of sampling and subsequent decay measurement. The result obtained,

14.23 ± 0.14 min, is in good agreement with results published by other

groups [Fri 6 1 , Ehr 72].

6ii12i_Y;r§Y._data_and_deçay__sçheme

143The decay scheme of JLa is discussed in paper [ V I ] , but neverthe-

less some comments on the decay of this nuclide wi l l be made here.143AY-ray spectrum of the La fraction is shown in f ig . 6 . 1 . This

spectrum is a sum of three spectra with a total measuring time of 90

minutes. From the relatively poor statistics of the spectrum i t is

understood that the absolute Y"intensities of the La Y-rays are

comparatively low. The spectrum is dominated by the Ce X-ray peak;

the strength of this peak probably arises from the emittance of con-

version electrons (as w i l l be discussed below, the (5-de cay of the143lowest-lying excited state in JCe is almost completely converted).

The partial decay scheme proposed for \ a is shown in f ig . 6.2.

In this figure, the levels obtained by Lessard et a l . [Les 72] are

shown as a comparison, together with the spin and parity data obtained

by this group.

66.

too»

. e 14al> sifif

EMcir w v o m iO « MV par CMMMI

S Î J ! si s 5 55 Eo •> - » r. " ! • : • !

J L L J J L L J

* s

5 ••

' " l . «inflw Htclrn.

fiwrnr r«»t» 1000 2000 k*V

Of ktV Mr ckaaiwl

3000

CHANNEL «UM«E«

cctrun of the '*'t« fraction. The spectrum was recorded on-line during 90 minutes.

The ground state of 'ce is spl i t up into levels at 0, 18.9 and14342.3 keV. For a nucleus like Ce, with three neutrons outside a closed

shel l , the spin of the lowest-lying member of the ground state t r ip le t

should be 1 - 5/2 according to the shell model. However, the spin of

this level has been measured to be 3/2 [Hal 65] . This anomaly can be ex-

plained by assuming that long-range Majorana forces are in operation

as suggested by Mai eh [Mai 65] . I t i s , however, to be expected that one

of the other members of the ground state multiplet is the 5/2-state.

The spin of a 21 keV state which is assumed to be identical with the

18.9 keV state found by us was measured as 7/2 in a polarization ex-

periment performed by Graw et a l . [Gra 69]. The parity of the ground state

and the 21 keV state was found to be negative.

67.

'oy<SS M«V

-f jffi;miii:::

y

»MO

-1MO4

IMTt

1727 1

£Z S

' ' 8=5 " ' *

«s ^ « 3

((•in .!«*•Il I I«! 711

-H71-2t*t

W M

U21 */2-

tit an'

Ml f/2-

iST 7J2-o afit

143C,

Figure 6.2. Partial decay scheme of La. The scheme is based on the coincidences listed in

table 2, paper [VI]. As a comparison, the levels obtained by Lessard et al. (Les 72] from

(d,p)-reaction data are shown to the right.

The experiments performed by us do not include any angular correla-

tion measurements, and we are therefore not able to present any mea-

sured spin of the 42.3 keV level , which we regard as identical with

the J»0 keV state reported by Lessard et a l . I t i s , however, possible

to make some conclusions about possible spins of this and other levels

from the decay scheme and the y-ray intensity data obtained.

Lessard et a l . [Les 72] found levels at 668 and 815 keVwith ln=9/2~

and 3/2", respectively. We considered the 815 keV as identical with the

817-2 keV state found by us and we also assumed that the 668 keV level

is identical with our 662.9 keV level. However, the energy difference

between the lat ter , 5.1 keV, is greater than the usual difference of

2 - 3 keV between our and Lessard's energy data. This difference makes

i t d i f f icul t to decide whether the levels are identical or not.

The 817.2 keV level is de-excited via a strong 798.3 keV y-ray

0 Y - 47.8 %) to the 18.9 keV state (AI « 2 ) , and via a somewhat weaker

68. J

fy-

77*.9 keVT-ray ( l y - 15.1 %) to the *2.3 keV state. This inplies that

the spin difference between the 817.2 keV and the 42.3 keV levels is

Al>2; thus ln>7/2~. On the other hand, the 9/2~ state at 662.9 keV

is de-excited via a strong 620.6 keV y-ray (I = 100 %) to the k2.3

keV level and a 6*3.9 keV y-ray ( I T = 71.6 %) to the 18.9 keV level

(AI = 1), which implies that the spin difference between the state at

662.9 and *2.3 keV is AI<1. This is contradictory to the spin value

obtained when considering the 817.2 keV level . There might, however,

be a simple explanation of this contradiction. Thus, from the Nilsson»A3

model the spin and parity of the La ground state can be determined as

7/2+ . I f this spin is correct, however, the ß-fee ding of the 662.9 and

8)7.2 keV states makes the spins assigned to these levels less probable.

Now assume that the spins assigned to the 662.9 and 817.2 keV states

are wrong or, which is more probable, that the levels found by us are

not identical with those of Lessard et a l . Instead, we suppose that the

662.9 keV is a 5/2" state and the 817.2 keV a 9/2~ state. I f this sup-

position is correct, i t explains the strong ß-feeding from the 7/2

ground state of La in an acceptable way (AI = 1 for the ß-decay to

these levels, which should give approximately equally strong ß-branches).

The spin 5/2 assigned to the 662.9 keV level is also supported by the

transition from the l/2~ state at 2255.* keV - i f the 662.9 keV level

was 9/2 , this transition would have AI = 4, which is not probable.

I f the above discussion is correct, the 42.3 keV state should be the

5/2 state expected from the shell model. The parity of the *2.3 keV

state is probably negative, since al l the members of the ground state

multiplet should have the same parity.1*3Before discussion of the spins of the Ce levels is closed, i t

should once more be enphasized that the assignments made here are not

measured data but conclusions based on the Y~ray intensity data.

The low-energy Y-ray spectra measured in the La fraction (cf.

f ig . 6.3*) give no indications that there is a 18.9 keVy-ray present,

although this y-ray should be one of the very strongest peaks. The ab-

sence of the 18.9 keV peak supports the spin assignment, 7/2 , to the

18.9 keV level; i f this spin value is correct, the transition from the

18.9 keV level to the ground state is mainly E2 and thus highly con-

verted. The 23.* and *2.3 keV transitions from the *2.3 keV level to

the 18.9 keV and the ground states, respectively, should then (assuming

69.

'»t.

Low «ntrgyY-fay tpMtrum

CHANNEL NUMBER

Figure 6.3. Low-energy f r a y spectrum of "•3 ,

that the 42.3 keV level has spin 5/2) be mainly Ml. This type of transi-

tion is also converted but the conversion should not be sufficient to

remove the y-ray peak from the spectra. In f ig . 6.3- » the 23.'» and 42.3

keVY~rays are present, as expected. This is further support for the

spin assignment 5/2 to the 42.3 keV state.

144La6.2. Data obtained for the decay of

144The nuclide La did not become accessible for nuclear spectrosco-

pic investigations until 1967 because of its short h a l f - l i f e . At that

time, Amarel et a l . [Ama 67] isolated the nuclide by on-line mass se-

paration. They also determined the ha l f - l i f e to be 41 ± 3 s by 3-ray

counting. No Y~ray measurements were reported.

Some years later, Ohyoshi et a l . [Ohy 72a,b] reported on the half-l i f e (42.4 * 0.6 s) and 9 Y-rays belonging to ' ^ L a . They isolated144 143

La in the same manner as \a [Ohy 71], i .e. by a fast electromi-gration technique.

In a study of short-lived Ba, La, Ce and Pr isotopes by means of

fast, automated of f - l ine chemical separations, Seyb [Sey 73] determined

the ha i f - l î f e of La to be 43 s. Seyb also reported four Y~ray ener-

gies at 397-2, 541.0, 585.0 and 844.8 keV.

Wünsch et a l . [Won 73] studied the Y~rays emitted from a mass-sepa-rated sample of the isobaric chain A >= 144. They found the three strongest144

La Y-ray peaks and also a Y-l ine at 260.9 keV.

70. J

The mass chain A » 144 has also been studied recently by Monnand et

a l . [Hon 74, Hon 76] at OSIRIS in Studsvik and LOHENGRIN in Grenoble.

These investigations resulted in a rather complete decay scheme, as

wi l l be discussed below.

144Our investigation of La, which was performed mainly in the years

1973-1975 includes accurate ha l f - l i f e measurements, determinations of

Y~ray energies and intensities and also y-T coincidence measurements

yielding a partial decay scheme. From the measured data, the log f t

values of the ß-branches have been calculated on the basis of certain

assumptions.

There are two reasons for the assignment to A « 144 of the 42 s

activity present in the La fraction. F i rst ly , h a l f - l i f e systematics ma-

ke i t probable that the 42 s activity comes between the 14 minute act i -

vity known (cf. section 6.1.1.) to belong to \a and the 25 s act iv i -145

ty that is attributed to ?La for reasons which wi l l be discussed in

the next section (6 .3 .1 . ) . Secondly, the measurement made by Amarel

et a l . [Ama 67] which was performed on a mass-separated sample gives

a reliable assignment of the 42 s activity to the mass 144.

The assignment has also been verified recently by the measurements

of Wünsch et a l . [Wün 73] and Monnand and Fogelberg [Hon 76]. Both these

groups measured on mass-separated samples, which makes the assignment144of the 42 s La activity to La unambiguous.

6.2.2 î ._Half :n£e

144The h a l f - l i f e of La was measured by the conventional technique

of sampling and subsequent decay measurements as well as the TDD tech-

nique. Both methods have, as expected, yielded the same h a l f - l i f e .

In the conventional measurements, two series of determinations were

performed. In the more precise series, we measured 8 x 2 K r~ray spec-

t ra , which gave an accurate h a l f - l i f e determination of the Y~ray peaks

below 2000 keV. The other conventional series, in which we measured

4 x 4 K spectra, allowed us also to calculate good half- l ives for the

strongest Y~ray peaks between 2000 keV and 4000 keV. The TDD measure-

ments were based on 4K spectra and involved 12 different delay times.

71. J

TDD measurements were performed also during the experiments performed

in Oslo (cf. paper [ I ] ) .

The h a l f - l i f e of the nuclide has been calculated as the weighted mean

value of the four most intense Y~ray peaks at 397.5, 541.3, 585.1 and

844.9 keV, respectively. The most accurate h a l f - l i f e determination

yielded 42.1 ± 0.7- Taking also into account the weak Y'J'nes, the half-

l i f e becomes 42.3 i 1.6 s. No Y~ray attributed to the decay of

differs more than 5 s from the mean value.

La

Fig. 6.4. shows a Y~ray singles spectrum recorded during a measure-ILL- 1/|£

ment on the nuclides La. I t covers the energy range 0 - 4000 keV.

The Y-ray peaks belonging to La as well as the peaks of ^La and La

are indicated. The latter nuclides w i l l be discussed in sections 6.3. and

6 .4 . , respectively.

Two independent Y~Y coincidence measurements have been performed on

the La fraction. The f i rs t one, which was performed at the Mainz Cock-

croft-Walton accelerator (cf. section 2.1.) yielded good data for La,

while the data obtained for ' " La were stat ist ical ly less significant

because of the long delay times in that experimental set-up. The other

coincidence investigation, which was performed at the TRIGA reactor, also

yielded excellent data for La and La. The results presented here

are based on al l SISAK data available.

Table 6 . 1 . shows the energies, relative intensities and observed144

coincidences of the Y~rays attributed to La. The decay scheme de-

duced from this data is shown in f ig . 6.5. To demonstrate the quality

of the coincidence spectra on which this scheme is based, f i g . 6.6.

shows the spectra obtained from the gates on 397.5 and 541.3 keV.

The f i rs t excited state in Ce is assumed to have l n • 2 + (the

one-phonon vibrational state) in accordance with the level sy sterna tics

for even-even nuclei. The energy of this state was determined as 397.5

keV, which is in agreement with the results of Cheifetz et a l . [Che 71]

who measured the energy of the 2 + •*• 0+ transition as 397-5 keV. The

level is assumed, from level systematics (cf. f ig . 6.7.) to be the 4

member of the ground state rotational band. This assumption is sup-

ported by the absence of any crossover to the ground state.

72.

2.10»

X

S

' * * ~ ' * * l J Singles spectrumEnergy fang« 0 - 1000 h »O.S k»V par channel

"a. 7 7777, « 3 3J3JÇ» nNnon

u

5 SSS? S 2 ÎÎ5

*> n m ^ « o

: s s ss: -

144 -14» La singles spectrumEnergy range-1000 • 2050 keVO.S keV per channel

144-146 L a singles spectrumEnergy rang« 2000 - 4000 heV1 keV per channel

^^^-X-JL3000

CHANNEt. NUMBER

Figure b.k, Y"ray spectrum of neutron-rich La isotopes, mainly " La. The spectrum was recordedon-line during 150 minutes.

73.

logft lp<%> S Sj• 0 2.3 32(3.2

iiiiiïi

397 5 2*

_ 0 0 0»144c.

Figure 6.5. Decay scheme of La. The scheme is based on the coincidences listed in table 7.1.

The 1102.7 keV level is probably the second 2+ state ( i . e . i t belongs

to the two-phonon t r i p l e t ) , since i t is de-excited via y-rays to the

ground state and the f i rs t 2+ state at 397.5 keV. This spin and parity

V*.

Table 6.1.Energies, relative intensities and coincidences observed for T-rays assigned to La

E,. (keV) Observed coincidences

367.3397.5

«32.15*1.3585.1597.9705.0735.<i752.18**.9952.79*9.0

1052.91092. li1102.*1276.71Z9i>.>>1*22.81*32.31*89.91523.91674.21820.319*3.31996.82003.02325.62865.7

2.9100.0

7.942.011.62.9* .8

11.33.3

28.17.58.2!.<•1.13.92.99.12.66.31.9<i.62.68.0.1.32.41.81.51.2

585. • (w)367.3(m),952.7<s),19*3.3 M397.5(s),367.3(w),367.3(i»),None397.5(s)397.5(s),None397.5(s),397.5(s),397.5U),397.5(n),397.5<m)None397.5(w)397.5(s)397.5 On)397.5(3)None397.5(s),NoneNone397.5(s)NoneHot gatedNot gatedNot gated

*32.l(s). 0*1.3<i), 585.I(s). 705.0(0. 735.*(s), 8**.9(s),969.0(5), 1052.9(w), 1092.*M, 1276.7U). 1*22.8(n), 1*32.3(s)., 2325.6(m). 2865.7M8**.9<s>. 969.0U)397.5(»), 5B5.1(s). 735.*(s), 952.7(s), 969.01s), 1052.9M. 2325.6(«)397.5(5), 5*1.3(s). 952.7(s)

5*1.3(s). 969.01s)

« 2 . U s ) , 969.0M5*1.3(s>. S85.l(s)432.1(5), 5*1.3(s), 735.Ms), 8**.9(s)* ( )

8**.9(s)

The uncertainty in the >ray energies is estimated to be ±0.5 keV below 2000 keV and * 1 keV above2000 keV. The uncertainty in they-ray intensities :z estimated to be *10Î for strong peaks(I > 10*1 and ±203: for rh» wpab nnp«.(I >10%) and ±20? For the weak ones.

assignment is also supported by the level systematics for the neigh-

bouring even-even nuclei (cf. f i g . 6 .7 - ) .

The level at 12*12.l» keV is de-excited via a 8^4.9 keVy-ray to the

2+ s ta te. From level systematics, i t seems reasonable that th is level

is the 3" state o r ig ina t ing from the octupole v ib ra t ion .

In an ear ly SISAK paper [ I V ] , we assigned spin and par i ty 6+ to the

1523.9 keV leve l . This assignment was based on the good agreement between

the experimental energy and the energy predicted by the variable moment

of i ne r t i a (VMI) model [Mar 69] , In a la te r paper [ V I I I ] , we expressed

doubts about the spin assignment because we observed a Y~ray wi th energy

1523.9 keV. We were not completely sure whether this y r a y is a ground

state t rans i t ion from the 1523.9 keV state or whether is de-excites a

higher- ly ing leve l . The second a l ternat ive seemed to be the more pro-

bable, since there was a weak coincidence between the 1523.9 keVy-ray

and the 397.5 and 8M.9 keVy-rays. A careful re- invest igat ion of the

La coincidence data has now made i t certain that the 1523.9 keV Y"

ray de-excites a level at 2766.3 keV. Thus, there is no crossover from

the 1523.9 keV level to the ground s ta te , which makes the assignment

of I n " 6+ to th is level s t i l l probable. Usually, such high spin states

75«

(00

76.

M 4 l «

Gala 3S7.S kaV

Compton corraction subtracted

Smoothad

1 MV par rtiannal

S S s: Si 52 SS

«M

01—

M« La

Gala 541 3 kaV

Campion correction nibtraclad

Smootriad

1 fcav par charmai

m

ï s^ . . J.(00

CHANNEL NUMIEN

Figure 6.6. Coincidence spectra showing the Y-rays in coincidence with the 397.S and S'il.} keV peaks.

are not populated in the 0-de cay, but i f the spin of La really is

4 (cf. the discussion below), a 0-branch to the 6+ state is possible.

The level at 1674.2 keV which is de-excited via y rays to the ground

state and the f i rs t 2 + , 4+ and 3~ states is probably a 2* level [Mon 76],

By using the Y-ray intensity balances and assuming a negligible 0-

feeding of the ground state, we have also been able to estimate the

intensities and log f t values for the ß-branches. I t is obvious that

the strangest 0-branches are those populating the 4* state at 938.8

keV, the 3~ state at 1242.4 keV and the 2+ state at 1674.2 keV. The

2 state at 397.5 keV is populated only weakly in the ß"-decay.

144From the intensity of the ß-branch to the f i rs t 2+ state in

the corresponding log f t value has been estimated to be >9.2. This im-Ce

'«et

1102

93» . - 1

2* 397 . -

3"

4*

2«

1190

1044.

454

Figure 6 .7 . Level systematics for the Z - 58 and N - 86 nuclei . The data have been taken from

the present work and refs.IChe 7 1 , Nuc 73, Hon 76] ,

plies that the parity of the ground state in La is negative and also

that the spin difference between these states is 1 or 2. From the strong

ß-feeding of thethe La ground state has l " » 3~ or k.n

and 3 states i t is also reasonable to believe thatSeeger [See 70] calculated the

l"-value of '""La to be either 1 or 6~, but neither of these valuesexplains the p-feed ing in an acceptable way.

The SISAK data on the decay of La are generally in good agreementwith data reported by other groups. Thus, a l l y rays observed by Ohyo-shi et a l . [Ohy 72a,b] and Seyb [Sey 73] have been observed in the pre-sent investigation. There is, however, disagreement with the resultsof Wünsch et a l . [Wün 73] concerning the existence of a 260.9 keV y-ray assigned to La by this group. Thus, there has never been anyindication of a y-ray at 260.9 keV in the SISAK y ray spectra of La.

77.

it,'-! • / •

144A decay scheme of La was recently published by Honnand and Fogel-berg [Hon 76]. This scheme is almost identical with the one shown inf i g . 6.5. t but there are some differences worth discussing. Thus, theSISAK data do not verify the existence of the levels at 1467.9, 1481.6,

1890.4, 1907.2 and 2694.4 keV. For some of these levels, this might be144due to the intensity of the La source.

The levels at 1890.4 and 1907.2 keV [Hon 76] were reported to bede-excited via 952.0 and 968.8 keVy-rays, respectively, to the 938.4keV state. In the present investigation, however, there is a coinci-dence between the 952.7 and 585.1 keV y-rays and also between 963.0and 432.1 keV. Thus, the 952.7 and 969.0 keVy-rays de-excite levelsat 2476.6 and 2643.2 keV, respectively. These levels were not reportedby Monnand and Fogelberg [Hon 76]. From the significance of the coin-cidences, we consider the existence of these levels as unambiguous.

The Y~ray energies measured in the SISAK experiments are in goodagreeemnt with the energies reported in ref. [Hon 76]. The differenceis usually less than 0.5 keV. The relative y-ray intensities also agreewithin the error l imits.

144Since the (L-value of La has been measured to be 5.3 MeV [Dev 76] ,

P

ß-feeding of levels above the 3263.2 keV level is energetically possi-

ble . Fig. 6.4. shows clearly that there are several Y~ray peaks above

2000 keV which have not been assigned to any La isotope. Probably, they144 146

belong to ei ther La or La;a def in i te assignment is not possible

because the counting s ta t i s t i cs did not allow good h a l f - l i f e determina-

tions of these peaks. None of these unassigned high-energy peaks is pre-

sent in any coincidence spectrum, which implies that they are ground

state t ransi t ions.

'La

78.

6.3. Data obtained for the decay of

144For La i t was possible to compare the SISAK results with results

from other research groups. These were obtained with several independent144methods, which places the La decay properties among the most rel iable

data presented in this thesis.

145Access to external data on the decay of La i s , however, far more

limited. Before our investigations began, Grapengiesser et a l . [Gra 70]

reported on a 25 s ac t iv i ty found In a mass separated sample with A «

145. The ha l f - l i f e was tentatively assigned to ^La. No y-ray energies

were presented. Seyb [Sey 73] determined the h a l f - l i f e indirectly as

28 ± 3 s. A similar value, 2<i i 5 s, was found by Fasching [Fas 70].

Recently the LOHENGRIN group in Grenoble reported on three Y-rays found

in the mass chain A = 145 which were attributed to La [Oev 76].

The SISAK investigation of JLa is so far the most complete. I t has

yielded the assignment of 18 y-rays to this nuclide. These Y-rays have

been f i t ted into a partial decay scheme including 10 excited states in

Ce. Although the absolute y-ray intensities of ^La are low, the

reliability of thebelow.

1*5,La decay scheme is good, as w i l l be discussed

§i3ili_i)ass_assignment

Our assignment of the 25 s act ivi ty to La is based on two reasons.

F i rs t ly , in measurements on the La fraction the h a l f - l i f e 25 s is also

obtained from the growth-and-decay curves of the y-ray peaks known to

belong to 3 min Ce (for the assignment of these peaks to Ce; cf .

section J.\.].). Such a growth-and-decay curve is shown in f i g . 6 .8.

Secondly, to confirm the assignments of the y-rays f i t into the decay

scheme we have performed a measurement of mass chain 145 at the mass

separator OSIRIS [Bor 71] in Studsvik. In the spectrum obtained, a l l

y-rays which show 25 s h a l f - l i f e could be identi f ied. This makes the

assignment of these y-rays (cf, table 6.2.) to La unambiguous.

144 145Like La, the h a l f - l i f e of ^La has been independently determined

both by conventional decay measurements and measurements according to

100 I»Growth •rri-Mcar tinw.l

Fiaure 6.8. Growth-and-decay of the 7Z4.3 keV peak of '*5Ce measured in the La fraction.

79.

145the TDD method. The determination of the JLa h a l f - l i f e was more

d i f f i cu l t because of the low Y-ray intensities and the location of the

"strong" ' ^ L a Y-ray peaks in the energy range 70 -200. keV where many

peaks l i e close together.

The f i rs t l l t5La h a l f - l i f e published by us, 20 ± 5 s [ I I ] , was based

on the decay of the Y-ray peaks at 118.4, 170.2 and 189.0 keV. The coun-

ting stat ist ics of these spectra (which were recorded during the Oslo

period) were rather poor, which explains the large error range and the

fact that this h a l f - l i f e later proved to be more than 5 s too short.

In the h a l f - l i f e determination performed at the Mainz TRIGA reactor

the counting statist ics were superior to the Oslo measurements, yielding

more accurate values. The h a l f - l i f e f inally assigned to La, 25.3 ±

2.6 s [ V I I I ] , »s a mean value of the half- l ives of the prominent peaks

at 70.2 and 170.2 keV. The strongest ' 5La peaks, i .e . 118.4 and 189-0

keV, were not used for the determination because they form multiplets

with other peaks.

All La peaks with I >20 % have been found to possess half- l ives

within the interval 25 ± 5 s. Fig. 6.9 shows decay curves for some1i|5La Y-rays.

o • 1M4 kW T1 /2 . » 1 1» • 701 tav T , a • 2f .0 s

40Dtcay limt. *

Figure 6.9. Decay curves of some 5la peaks.

80.

Table 6.2. shows the yrays attributed to the decay of La. The

table also shows relative Y-ray intensities and observed coincidences.

The corresponding decay scheme is shown in f ig . 6.10. All Y~rays with145

I >10 % are known to belong to La not only from h a l f - l i f e and coin-

cidence relationships but also from the previously mentioned experiment

at the OSIRIS mass separator.ilr

The only other group which has presented Y~ray energies of La is

the LOHENGRIN collaboration. They reported on Y~rays at 104.0, 151.5

and 229.1 keV [Oev 76]. None of these lines has ever been observed by

us. This might be explained by the low intensity of the samples obtained

at LOHENGRIN, which increases the chance of making incorrect assignments.

Probably the Y~rays reported belong to either Cs or 8a.Table 6.2.Energies, relative intensities and coincidences observed for y-rays assigned to "*\a.

. IfceV) . tt> »bserved coincidences

48.270.2

117.1118.1.165.3170.2189.0215.3254.0«03.7799.88 to. 9693.2918.2932.3959.2

1053.21167.8

UO. 267.4<5

10D.010.138.5S*.*14.37.0

19.53«.7«5

49.453.811.2<5

15.9<5

165.3(5). 799.80»), 8 4 0 . 9 M . B90.2U). 932.3(s)Not gated2l5.3(s) . 403.7(5), 799.8U), 840.0(s)70.2U) , 2 1 5 . 3 M . 403.7<s>254.Ota) -165.3M. 215.3M189.0(w), 254.0(n)170.2W, 215.3(i.)70.2U), 117.K»), 118.4(5), I65.3(s)70.2M, 118.Ms)118.4M70.2(s)None70.2(s), 118.4(5). 165.3(w)Not gated70.2 MNot gated

The uncertainty in the Y-ray energies is estimated to be i0.5 keV.The uncertainty in the y-ray intensities is estimated to be 420*.

6.4. Data obtained for the decay of 1Z|^La

In 1971, Cheifetz et a l . [Che 7U presented data from four parameter

coincidence measurements (including Y-rays, X-rays and fragment masses)

on products formed in the spontanuous fission of 2 5 2Cf. They assigned

the Y"rays found at 502.3, 410.1 and 258.6 to the transitions belonging

to the 6+-»4+->2+->-0+ cascade in 1/>6Ce. Their investigation did not include

any ha l f - l i f e determination, so the only ha l f - l i f e value available when

we started our investigation was 15 * 10 s , measured indirectly by

Fasching [Fas 70]. During our investigation, a more accurate ha l f - l î f e

was reported by Seyb [Sey 73] who obtained 8.3 s. Recently, the Y-ray

31. J

T W - ï i . , »

~*9

s ""

» ! «•>

9 "

s 2w 0

s s

- 11»7i. 11234

•5*2

SU 2

63S.2

T 6 Z. —• _' - » « K

235.5

. . . - 1702

702

0.0

11

Figur« 6.11). Partial dtcay scheme of ' * S l a . The icheme is based on the coincidences listed Intabl* 6.2.

energies of La have also been determined by the LOHENGRIN collabora-

tion [Dev 76].

Our investigation of La has yielded a partial decay scheme as

well as Y-ray intensities and an accurate h a l f - l i f e . So far , our decay

scheme is the only one available for La.

82.

Originally, the assignment of the ~10 s activity found in the La

fraction was based on the excellent agreement between our Y-ray energies

(258.5, A10.0 and 503.2 keV) and the energies reported by Cheifetz et at.

J

[Che 71]. Later on we fe l t that such a mass assignment was not unambiguous,

and therefore we performed a growth-and-decay measurement in which we

studied the growth of the most intense y-ray peaks of Ce. This mea-

surement yielded the same ha l f - l i f e for La as the measurement of

the decay of individual peaks.

The assignment of the 503.2 - '»10.0 - 258.5 keV cascade to the decay

of La is also strongly supported by the level systematics for the

Z = 58 and N = 88 even-even nuclei (cf. f i g . 6.15).

Recently, the A = 146 mass chain has been studied at the LOHENGRIN

mass separator [Dev 76]. This investigation makes the assignment of the

f-rays l isted in table 6.3. to the decay of La unambiguous.

6.4.2. Ha l f - l i fe

146,The f i r s t h a l f - l i f e measurement on La was performed during the

experiments in Oslo. In that measurement, which allowed us to deter-

mine the h a l f - l i f e o f the 258.5 and 410.0 keV y-rays to be 11 ± 1 s

[ I ] , we used the TDD technique [ I I , Aro 74] . The h a l f - l i f e of the wea-

ker Y" l ines was not measured due to the low source strength.

During the experiments in Mainz, the h a l f - l i f e of La was measured

by the TDD as wel l as the t rad i t iona l technique. The increased produc-

t ion rate now also allowed us to determine the ha l f - l i ves for several

other Y"rays. Some of the decay curves obtained are shown in f i g . 6 .11.

In these curves, there is an obvious tendency for a l l the y-rays to

have a longer- l ived " t a i l " , and also for the y-rays ly ing high in the

decay scheme (cf. f i g . 6.14.) to apparently contain more of the long-

l ived component than those in the bottom. From old measurements, the

h a l f - l i f e o f th is long- l ived component was determined as 4.5 min (cf .

the decay curves in f i g . 6 .12. ) . Taking th is component into account,

the h a l f - l i f e of La becomes 8.5 ± 1.0 s , in good agreement wi th the

8.3 s obtained by Seyb [Sey 73] and the 8.0 ± 0.8 s recently obtained

by the Grenoble group [Dev 76] . The existence of the long- l ived com-

ponent is confirmed by the decay curves o f Seyb, which also show " t a i l s "

wi th >2 min h a l f - l i f e [Sey 76] .

Since th is long-l ived component has been observed for at least s i x

of the La Y"ray energies, i t can probably be concluded that i t does

not orginate from another nucl ide. Instead, there must ex is t an iso-

83.

r

figure 6.11. Decay curves of some La peaks.

woo

100

10

I

Tf/jlitiMn) < 4 . 1 * 1.2 min

0 • 2SI.S UV T• • 410.0 k«V 1

T

'V2 ' 4.4 11 2 min1/2 • 4 f t 2.0 min

1»O«e*y tiim.mi»

Figure 6.12. Decay curves of the 258.5 and 410.0 keV T-rays Indicating the existence of anIsomeric state in '^La.

meric state in La. This isomeric state is also indicated by the al-

most equally strong ß-feeding to 2 + , 3~, A+ and 6+ states in Ce.

8*..

Such a &-feed ing would not be probable unless there is an isomeric state.

L I/.Ç

La, La and La was shown inA y ray spectrum of the isotopes

f ig . 6.k. Like La, the nuclide La possesses y-rays in a wide ener-

gy range, from 183.3 keV up to 3*125.0 keV. Above 2000 keV, there are

probably many peaks belonging to La, but the low intensity of these

y-rays has hindered us from making may definite assignments.

Table 6.3. shows the Y~ray energies of La and also the correspon-

ding relative Y-ray intensities and observed coincidences. From these

data, the partial decay scheme shown in f ig . 6.13. has been deduced.

Fig. 6.\k. shows some of the coincidence spectra on which the decay

scheme is based. The f i rs t excited state in Ce is the level at 258.5

keV. As Ce is an even-even nucleus, i t should be the one-phonon vib-

rational state (I = 2 ) . From the low energy of this state, i t is pro-

bable that Ce is considerably deformed.

That the level at 668.5 keV is the k+ state belonging to the ground

state rotational band can be unambiguously deduced from the level syste-

matics (cf. f ig . 6 .14 . ) . This l^-assignment is supported by the absence

of any crossover to the ground state.

The next excited state, 92^.9 keV, is probably the 2+ level belonging

to the two-phonon t r ip le t . This assignment is supported by the de-excita-

Table 6.3.Energies, relative intensities and coincidences observed for Y-rays assigned to la.

E (keV)

183.3258.5

292.4368.3380.1410.0446.8503.2515.0666.4702.4785.4924.9959.1

1018.01043.41141.91810.42065.02291.52379.52612.63397.53656.0

l_ (*)T

7.B100.0

1.14.05.0

62.914.921.322.47.5

15.24.14.26.4

2.88.9<14.96.52.71.91.3

Observed coincidences . '

258.5(s).183.3(s),515.01s),2612.6MNot gated2 58.5 W .258.5(5).183.3(0.l«3.3<«).25B.5(s).258.5(s),183.3(i»),183.3(").258.5(5).None183.3(s),258.51»)None258.5(s) ,Not gatedNot gated258.5(5)1018.0(w)785.4M258.5(5)Not gated

292.4(w),292.4(m),666.4(5),

410.0(s), 446.8(m), 702.4(s)368.3(m|, 38O.Ks). 4l0.0(s). 446.8(s), 503.2(s),702.41s), 785.4(s). 959.Us), 1018.Olw). 1141.9(s).

, 3397.5(s) • ••

410.0(s).410.0(s),25».5(s).25«.5<m),380.1(s),368.3M,258.5(s)258.5(s).2612.6(m)

258.5(s),

410.Oil)

515.0(s)503.2(s)292.4(m), 446.8(m), 503.2(s), 5IS.0(s), 959.Km)«10 .0W,959 .Ks)410.0(s)410.01s)

446.81m)

446,8(s), 702.4(s)

Ttie uncertainty in the ï-rey energies is estimated to be 10.S keV below 2000 kell and ±1 kevabove 2000 keV. The uncertainty in the Y-ray intensities Is estimated to be ±10% for strongpeaks d T >10i) and *203 for the weak ones. ^

85.

p • •

•"»' » » I• • I I I I I

70

4 *

27

«0

SS3 "

z •

3«M.O

25S01

1810 4

15*1015618

«P! 1276 511836117171144210434

S • 980 93 924 9

(V.2'1

(3")2«

6*8 5 4*

258.5 2*

86.

Flour« i. 13. Partial dtcay » d i m oftabla 6.3.

«. The scheme Is based on the coincidences listed In

tion of the 924.9 keV state via a 924.9 keV Y~ray to the ground state

and a 666.4 keV transition to the 2+ state at 258.5 keV.

At 1043.4 keV there is a state which might be either a l" or a 2 +

state. From the information available, i t is impossible to decide which

spin should be assigned to the level.

The level at 1171-7 keV is probably the 6 state belonging to the

ground state rotational band. I t was already assigned lïï=6+by Cheifetz

et a l . [Che 71] in his investigation of ground state transitions.

Our assignment of spin 6+ to the 1171.7 keV state is based on level

sy sterna tics (cf. f i g . 6.15.) and the excellent agreement between the

experimentally observed levels and the levels predicted by the VMI

model [Mar 69].

Git« 25» 5 keV

Compton correction subtracted

Smoothed

1 keV par channal

£ 250

S

" « L a

Gata 410.0 fceV

Complon correction subtracted

Smoothed

1 keV per channal

CHANNEL NUMBER

F i g u r e 6 . H . C o i n c i d e n c e s p e c t r a v.howinn, the Y - r a y s in c o i n c i d e n c e w i t h the 2 5 8 . 5 a n d *tiO.O k e V p e a k s .

87.

I»M W l

.19» 2*

4-*s?—

2-

104t

773

334

figure 6.15. Level systematics for the Z =• 58 and N * 88 nuclei. The data have been taken from

the present work and refs.[Bow 70, Che 71, Nuc 73],

As was the case for the decay of La, the intensities of the ß-

branches from the decay of La have been calculated on the assump-

tion that the ground state of Ce has a negligible or at least weak

ß-feeding. There has been no possibi l i ty to check the val id i ty of this

assumption, but i t seems reasonable. Thus, the ß-feeding of the ground142 ]UU

states of the other even-even Ce nuclei , Ce and Ce, is low (<10 %),

Furthermore the high y i n t e n s i t y of La makes i t improbable that

there is any strong ß-branch directly to the ground state of Ce.

The ß-intensities obtained show a strong ß-feeding to the 6+ state

at 1171.7 keV and the 3" state at 1183.5 keV. There is also a consi-

derable ß-feeding of the 2+ state at 92^.9 keV and 4+ state at 668.5

keV. I t is obvious, that states with spins as dif ferent as 2 + , 3", k*

and 6 cannot be almost identically populated by ß-de cay from one single

88. J

state. Thus, i f these spin assignments are correct, there should exist

an isomeric state in La. The evidence for this isomer is weak be-

cause the assignment of spin 6 is based only on nuclear level syste-

matics and the consistency with the phenomeno logical VMI-model. The

existence of the isomeric state is however, supported by the 4.5 min

component found in the decay curves of La y rays (cf. section 6 .4 .2 . ) .

From these curves and the decay scheme i t also seems probable that the

8.5 s component originates from the decay of the La ground state

and that the long-lived component comes from the decay of the isomeric

state. This statement is supported by the fact that the long-lived

component is relatively stronger in the decay curves of y-rays de-

exciting presumed high-spin states. According to the calculations made

by Seeger [See 70], the spin of the l6

2

}l*6La ground state should be 1 +

or 2 . I f these figures are correct, the spin of the isomeric state

might be between 4 and 6.

In comparison with the decay scheme published by us in ref. [ V I I I ] ,

the scheme shown in f ig . 6.13. contains one difference. In that paper,

the 666.4 keVy-ray was believed to de-excite the 1810.^ keV state. A

new investigation of the data available on La showed, however, that

the 666.4 keVyray is the transition from a presumed second 2+ state

at 924.9 keV to the 2+ level at 258.5 keV. The y r a y from the 924.9 keV

level to the ground state has also been identified.

6.5. Data obtained for the decay of \

When the new SISAK 2 system had been installed at the Mainz TRIGA

reactor in early 1976, the f i rs t experiment was a test of the rapidity

of the new equipment with the chemical systems used for isolation of

La, Ce and Pr. For La, these experiments yielded the f i rs t separation

of the two previously unknown isotopes 'La and La. These nuclides,

with half- l ives 2.2 and ~1 s, respectively, are so far the most short-

lived nuclides studied by the SISAK technique.

The 2.2 s La activity has been assigned to mass 147 because there is

an excellent agreement between the half-lives obtained in measurements

of the individual y-rays and the growth-and-decay of peaks belonging to

Ce. Fig. 6.16. shows such a growth-and-decay curve of the most pro-

89.

£?•

minent ^Ce peak at 269.1 keV. For reasons that w i l l be discussed in

section 7.3. i t is a l i t t l e risky to use the 269.) keV peak for these

purposes, since i t is influenced by the 269.7 keVy-ray from Ce.

However, the ratio between the areas of these peaks is approximately

4:1 and so i t was considered possible to use the low-energy part of the

269 keV doublet for the growth-and-decay measurement.

Recently, the Grenoble group obtained data [Dev 76] which confirm147our asjignments of Y'rays to La.

100 130

Growth • and'dacay tima.s

"•"" 6-'6- <s™w«-n«-<»ee.y curve of the 269.) keV peak of " " c e measured in the la fraction.

90.

147The h a l f - l i f e of La was determined by means of the conventional

technique of sampling and subsequent decay measurements. In the mea-

surements, a time sequence of 4*2, 5x8, 3x20 and 4x40 s was used;

altogether 16 spectra with IK channels were measured. Thus the measure-

ments allowed détermination of half- l ives between 1 s and ~40 s. 110

cycles were accumulated to achieve good counting stat ist ics. The peaks

assigned to La showed a ha l f - l i f e of 2.2 ± 0.4 s. Decay curves for147some of the La r~rays are shown in f ig . 6.17. The ha l f - l i f e obtained

in the S5SAK measurement is a l i t t l e shorter than the one recently mea-

sured by the group at LOHENGRIN (4.0 ± 1.0 s ) , but considering the low

Intensity of the

as consistent.

147, La sources available, the values may be regarded

r r

figure 6.17. Decay corves of some La peaks.

Fig. 6.18. shows a y ray spectrum of the most short-lived '.a isoto-pes. The spectrum was obtained by subtracting the third and fourthspectra recorded during the decay measurement (measured from k - 8 s)from the f i r s t two spectra (0 - k s). In this manner, the influence of"long-lived" act iv i t ies , i .e. half- l ives greater than ~10 s was sup-pressed.

\klThe y-rays of La are listed in table 6.k. The assignment of these147Y~ray energies to La is based on the half- l ives. For most peaks, i t

was possible to determine an acceptable ha l f - l i f e , but for the weak

peaks and for those belonging to multiplets, the assignment was based

147 14B*" ' " ™ " L * iingiti spectrumEnergy rangs O 1OOO fceV

SS SS'oï S3 S

500CHANNEL NUMBER

1000

Figure 6.18. T"ray spectrum of the roost short-lived La isotopes. To obtain this spectrum, the th i rdand fourth spectra of a decay measurement (measured *-8 s after start of the measurement) «eresubtracted from the f i r s t two spectra (measured O-* s) .

91.

m ' •

on another criterion: if the peaks were short-lived (i .e. not belonging144-146

to La) and were present up t o ~ 8 s after the start of the msa-

sûrement, they were assigned to 'La. I f they disappeared wi thin 4 s ,

they were tentatively assigned to La.

The y-rays obtained by the Grenoble group [Dev 76] in their investi-

gation of the A = 147 isobar includes the Y-rays found by us and also

some more (probably the influence from the Ce and Pr daughters has,

in this specific case, been less disturbing than the influence from144 146

La and La in our chemically separated La fract ion). The agree-

ment between StSAK and LOHENGRIN y-ray energies is excellent, usually

within 0.3 keV.

6.6. Data obtained for the decay of La

In the f i rs t experiment with the new SISAK 2 equipment on the La

fraction, we were also able to identify La. This is probably the

most short-lived neutron-rich La nuclide we wi l l ever be able to study,149

since the fission yield of ^La is too low (only 0.1 % according to

ref. [Wea 63]) to allow i t to be studied in the same fraction as l W " ' * 6

149La is probably also too short-lived (<0.5 s ) .

The assignment of the ~1 s act iv i ty present in the La fraction to

La is based en the good agreement between these y-ray energies and

the differences between the levels in Ce. These levels are known

from the study of prompt y-rays from fission fragments performed by

Cheifetz et al.[Che 71] . We have not been able to verify the mass

assignment e.g. by growth-and-decay measurements because the time

sequence used was not aimed for half- l ives ~2 s. Cne of our La

Y-rays, 158.6 keV, has been observed by the Grenoble group tOev 76] in

a measurement of A = 148.

92.

As mentioned above, the measuring time sequence used did not al low

determination of half- l ives ^ s. However, by studying the two f i rs t

decay spectra (recorded 0 - 2 and 2 - 4 s after the start of the mea-

surement) and with knowledge of the delay properties of the SISAK 2

system the h a l f - l i f e of 11|8La can be estimated to ~1 s. This value is

much shorter than the one published by Fasching [Fas 70] , who measured

the h a l f - l i f e as 20 i 6 s in a growth-and-decay experiment. Probably,

Fasching measured another nuciide ( La?) because i f the h a l f - l i f e of

LLa is 20 s, we would have seen the La peaks in al I the decay

spectra. In the y-ray spectrum shown in f i g . 6 .17 . , the La y-rays

have been indicated.

No y-y coincidence measurements have been performed on La, but

nevertheless i t has been possible to f i t most of the y-rays into the

level scheme previously mentioned. Thus, i t was possible to identify

the k*-*2* and 2 -»0 transitions belonging to the ground state rotatio-

nal band (295.8 and 158.8 keV, respectively). The transition from the

second 2 + level at 537-7 keV to the ground state can also be identified

in the spectrum, while the presence of the 378.9 keV y-ray from the

2' -*2 transition is more doubtful, since this y-ray energy is situated

close to the 380.1 keV y-ray from La.

A partial decay scheme for La is shown in f i g . 6 .19- , and the y -

rays of La are also l isted in table 6.6. Fig. 6.20. shows the level

systematics for the Z = 58 and N = 90 nuclei in the neighbourhood of

Ce, supporting the spin and parity assignments.

537 7 C2*>

454 6 4-

158 5 2*

U8,Figure 6.19. Partial decay scheme of °La. The scheme is based on the agreement between the energies

the most short-lived T-rays found in the SISAK exper

'Ce levels reported in ref.[Vil 69, Che 71, Non 76J.

?fothe ™ost s n o r t~ 1 i v e d T-rays found in the SISAK experiments and the difference b- t K = c n the

93.

r

310

O O' 0

'"o

Figure 6.20. Level systematics for the Z • 58 and N - 90 nuclei. The data have been taken from ttm

present work and refs.[Che 71. Nuc 73, Hon 76].

Table 6.li. T-ray energies of 1ll7La and l l l 8La.

7L» T-r«y energies, keV I* Y-**T energies, keV

117.9

U7 . I

21S.5

235.3

273.9

263.6

3S3.2

382.2

399.7

43B.6

507.5

516.8

571.0

158.5

295.8

378.9

S37.7

i Ir7. DATA OBTAINED FOR THE DECAY OF NEUTRCN-RICH Ce ISOTOPES

Knowledge about heavy La isotopes was, as discussed in the previous

section, rather limited at the time when this investigation began. One

reason for the lack of nuclear data was that most of the La isotopes at

the neutron-rich side of the stabi l i ty valley are too short-lived to be

studied without fast and sophisticated methods. For Ce, however, the

situation was different. The Ce isotopes with A <146 have half- l ives

>3 minutes and they are therefore easily accessible to e.g. conventio-

nal of f - l ine chemistry. The nuclides 'Ce and Ce have half- l ives of

~1 minute and should also be possible to study by a fast of f - l ine tech-

nique. Nevertheless, the data reported on Ce isotopes was confined to

the investigations made by Hoffman et a l . [Hof 64, Hof 65, Hof 66 and

Hof 68] and Ohyoshi et a l . [Ohy 72a and Ohy 72b]. The investigations145performed by Hoffman et a i . were concentrated on 3 min Ce and 14

min Ce. The authors also reported on the half- l ives of Ce and

Ce. To isolate a pure Ce fraction, Hoffman et a l . used J plastic

column on which HDEHP had been adsorbed. I t worked analogously to our

column described in section 3 .4 .3- . i .e. tetravalent Ce was selectively

extracted.

Ohyoshi et a i . made a study of Ce and, part ia l ly .of Ce by means

of a fast electron»gration technique. This investigation i s , however,

not comparable with the thorough study made by Hoffman et a l . , since no

Y~Y coincidence measurements were performed.

The Y~ray data published on Ce is so far limited to results

obtained in the SISAK experiments. These results wi l l be discussed in

this section together with some unpublished results.

Recently, the group at the LOHENGRIN mass separator in Grenoble ob-

tained data on the neutron-rich Ce isotopes [Dev 76]. These results in-

clude hal f - l ives, y-ray energies and relative intensities.

145Ce7-1 . Data obtained for the decay of

145The nuclide Ce was f i rs t reported by Markowitz et a l . [Mar 54] ,

who separated i t froir a fission product mixture by a solvent extraction

procedure. The ha l f - l i f e was determined as 3.0 min by ß-ray counting.

145No thorough investigation of Ce , however, was performed until

Hoffman et a l . started their study of 5Ce and Ce [Hof 64, Hof 65,

95.

Hof 66]. They published a decay scheme based on T T coincidence measure-

ments with Nal (Tl)-detectors.

In 1972, Ohyoshi et a l . [Ohy 72a] made a re-investi gat ion of Ce

by Ge(Li)-detector measurements. This group also presented a decay

scheme that was identical with the scheme published by Hoffman et a l .

except for one level . The main reason for the difference was probably

that Ohyoshi et a l . did not base their scheme on coincidence measure-

ments, but only on the sums of y r a y energies.

i Ir

The assignment of the 3 min Ce activity to Ce is now so well

established, that we have found no reason to question i t . Thus, the

3 min Ce activity has been observed not only in experiments based on

chemical separations, but also at mass separators [Gra 7h) which mokes

the assignment orginai ly made by Markowitz et a l . [Mar 5*»] unambiguous.

The h a l f - l i f e of Ce was determined only by the conventional tech-

nique of sampling and subsequent decay measurements because the long

h a l f - l i f e is less suited for TDD determinations. During the measurement,

Ce was adsorbed on the HDEHP/PVC column described in section 3 .4 .3 . ,

while grown-in Pr (mainly Pr) was continuously eluted to de-

crease the amount of less interesting information in the y r a y spectra.

The h a l f - l i f e obtained, 3*0 ± 0.1 min., is in excellent agreement with

other 5Ce h a l f - l i f e data [Mar 5h, Hof 65]. The short-lived isomeric

state suggested by Seyb [Sey 731 was not observed.

96.

Ziii2i_YlEaY_âa.£a_a.n.d__d.e.£a.Y._s_£h.Ême.

During the experiments preceeding this thesis, Y'Y coincidence mea-

surements on Ce were performed on two occasions. Both times, Ce

was adsorbed on a HDEHP/PVC column that was changed every 30 minute to

decrease the influence from the Y" in tense nuclide Ce. A Y" ray singles

spectrum of the Ce fraction is shown in f i g . 7 .1 . In this spec-

trum, which was recorded during a run at the Mainz TRIGA reactor, al l

known Y~ray peaks belonging to Ce are indicated together with

the major contaminants. The spectrum clearly demonstrates that the Ce

fraction is extremely pure.

r

145 1 4 * c . „ n g , e , s p K ( r u m

Energy »•«»• 500 1000 k«V

0 S fceV p*r channel

« IDSS

10000

0

•

•o

* ^

—r

•u

«o

— - • ! 1 • ' 1 •

145 148ca singles ^»ctrotn _

Energy range 1000 1500 K«V

0 5 K*V per chant»!

<

— 1

294

2500

CHANNEL NUMBER

Figure 7.1. T-ray spectrum of neutron-rich Ce isotopes, mainly ^~ Ce. The spectrum was

recorded on-line during 210 minutes.

97. J

r r« • • » • ! = • «

S S

1

•w w ® ^ ^ ^

8 = 8 5 5 3

O r•0 CO) 1>n < ? S

-jp> fit -ggi

O N (0•- S *in « CD

S

1

1ZW7

8P96

7871

5SS0

35103474

62700

14Sp,

Figure 7.2. Partial decay scheme of Ce. The scheme is based on the coincidences listed in

table 1, paper [IX].

The decay scheme proposed for 5Ce is shown in f i g . 7-2. I t is based

on the coincidences l i s t ed in [ I X ] . A comparison o f the decay scheme

derived by Hoffman e t a l . [Hof 66] and the one obtained by us shows that

a l l levels and Y" trans i t ions found by Hoffman agree wi th our data. In

addit ion to the levels found by Hoffman, we have a level at 555-0 keV

that is de-excited v i a y - r a y s to the Ikl.k and 62.7 keV levels and the

ground s ta te . The 555-0 keV level is fed via a 232.0 keV y-ray from the

787.1 keV level and a 655.3 keV t rans i t ion from the 1210.7 keV leve l .

The reason why Hoffman did not f ind the 555.0 keV level is probably that

the y r a y s feeding and de-e.xciting i t were too weak to be observed.

In the present decay scheme, the level at 350 keV proposed by Hoff-

man et a l . and Ohyoshï et a l . [Ohy 72a] has been s p l i t up into two l e -

vels at 3*«7.h and 351.0 keV, respectively. Such a small energy d i f f e r -

ence could not be observed by Hoffman, who used s c i n t i l l a t i o n detectors.

98.

rThe decay scheme proposed by Ohyoshi shows a level at 300 keV, fed

by a 492 and a 915 keVy-ray and de-excited via 233 and 300 keV y-rays.

From the SISAK data, we have not been able to verify the existence of

the 300 keV level , even though our Ce source strength was higher than

Ohyoshi 's. The y-rays at 233 and 492 keV are probably identical with

the v-rays found by us at 232.0 and 492.2 keV. As indicated in f ig .

7 .2 . , we have connected these y-rays with the 555.0 keV level.

A particular problem arising in connection with the evaluation of

Ce data was the calculation of relative intensities for the 351.0

and 351.1 keV y-rays. These energies are so close together that they

are impossible to resolve. Furthermore, Ce has a y-ray at 352.1 keV

also influencing the area of the doublet. However, the exact ratio146between the areas under the 352.1 keV line of Ce and the 351 keV

145doublet of Ce, respectively, was obtained from the Y-axis intercept

in decay curves of long-lived Ce isotopes. Thus we were able to calcu-

late the contribution from the y- l ine orginating in the decay of Ce.

The intensities of the members of the doublet were then calculated from

a coincidence spectrum (gated on 351 keV). In this spectrum we compared

the areas of two y-rays of known relative intensity, one known to be

coincident with the 351-0 keV l ine, the other with the 351.1 keV l ine.

This allowed us to calculate the ratio between the members of the doub-

let by making one important assumption, viz. that no isomeric states

are involved; i f there are, the calculation is erroneous. Due to the

errors involved in this I -determination, the intensities of the 351.0

and 351*1 keV y-ray have to be regarded as less reliable than the other

intensities.

145CSince Pr is an odd-even nucleus, there is no sense in trying to

e grou

5/2",

discuss possible spin and parities of this nucleus. However, the groundl1f 3/2+ and the ground state of 1i|5Ce l"state should have I

according to the computation made by Seeger [See 70].

The levels at 787.1 and 1210.7 keV seem to be the most strongly fed

in the 3-decay of Ce, while the other levels have an almost negligible

ß-feeding. From the y-ray intensity balance, i t is also obvious that

the 62.7 keV y-ray de-exciting the lowest excited state of Pr is

highly converted.

99.

r r7.2. Data obtained for the decay of Ce

In 1943, Hahn and S trass mann [Hah 43] reported on a 14 min Ce iso-

tope prec ip i ta ted from a f i ss ion product mixture. The a c t i v i t y was146assigned to Ce. This nuclide was not studied again u n t i l Hoffman et

a l . began the i r invest igat ion about 20 years la te r . The measurements

made by Hoffman [Hof 68] included y-y and ß-y coincidences, which f a c i -

l i t a t e d the proposal o f an almost complete decay scheme. The authors

also made a careful discussion of possible spins and pa r i t i es o f the| ! f 6Pr levels fed in the decay of | l | 6Ce.

The h a l f - l i f e o f the most prominent Ce Y~rays was then checked

by Fasching [Fas 70] and Ohyoshi et a l . [Ohy 72a], who achieved the

same value as Hoffman et a i .

Although «.he decay propert ies o f Ce were wel l established when

we star ted our Ce experiments, we checked the decay data published on

this nuclide since we obtained strong Ce sources in addit ion to Ce,148,.147Ce and Ce.

146,With regard to te, there is no reason to discuss the mass assign-

ment of the 14 min a c t i v i t y or the h a l f - l i f e , since these propert ies are

wel l established from several independent measurements [Hah 43, Hof 68,

Fas 70, Ohy 72dj. The h a l f - l i f e measured in the SISAK experiments,

14.2 ± 0.3 min, is in good agreement wi th the previous values.

The results of the Y~Y coincidence measurements on Ce are l i s t ed

in table 7 . 1 . From these data, the decay scheme shown in f i g . 7-3. has

been deduced. This scheme is based on a new examination o f the co inc i -

dence data, y ie ld ing one more level compared to the scheme published

in [ I X ] .

As mentioned in paper [ I X ] , the agreement between our decay scheme

and the one proposed by Hoffman et a l . [Hof 68] is exce l len t , except

for the Y-rays at ~360, 467.9 and 491 keV which were not observed in

the present invest igat ion. Another difference is that we were able to

ver i fy the existence of a 35.0 keV y r a y de-exci t ing the 35.0 keV le -

vel proposed by Hoffman et a l . [Hof 68] .

In réf . [Hof 68 ] , Hoffman et a l . suggest that the 35-0 keV level is

100.

rrTable 7.1. Energies, relative intensities and coin;idences observed for r-«-ays assigned to Ce.

C (UV)

35.052.265.367.098.7

101.0106.Jr33.7•<il.5210.7218.5251.22M.9317.1352.'369.8«•15-9503.1

5.2Ï . 0J.7

92.6* . *0.3

JI.B13.118.3'1.310.03B.3

100.00.65.*5.56.2

2S«.2ïsl«oneZl9-5fsJ2)O.7(«)1*»1.5U1. «06.3M9S.7(4i. 133.7(*JI0l.01t1S2-2(s}. 65-3<«), 67.d{*l35.0(s)None

9B-7M. 133-7' }«oneNone

maSec to fee »0.5 keV. The uncertainly in th« r-rayinteniitie-peats ( ' :

k is et t i»•10JI and

lated ÏO toe »11V for ï t rçng«20? fur ine «tak o n » .

de-excited via coincident y rays of ~12 keV and ~23 keV, respectively.

Thus there could be a level at either 12 or 23 keV. On the basis of a

weak kS) keVy-ray de-exciting the 503 keV state, Hoffman et a l . pro-

posed the energy of the level to be 12 keV. However, in the present re-

investigation, the existence of the 491 keV y-ray was not confirmed.

Furthermore, there is a coincidence between the 26'».9 keV y-ray and

a 65-3 keV y-ray. The 65-3 keVy-ray de-excites the 87.0 keV state,

and therefore there must be a level at 21.7 keV. This level shou.'d be

identical with the 'eve! placed at ~I2 keV by Hoffman et a l .

Because of their low energy, the 21.7 keVy-ray and the expected

T i /2 •

"3 -n "3 'S 'W

s 3 B Iz503 1

s; o o

io m r. r«. en w o

-141 5-133 7-1010- «70.-35 0- 2 1 7

00146 pr

CFigure 7.3. Partial decay scheme of Ce. The scheme is based on the coincidences l isted in

table 7.U

101,

r13-3 keV Y~ray could not be observed in any Y-ray spectra. The 65-3 keV

Y~ray, however, appears both in Y"Y coincidence and Y-stngles spectra

(cf. f i g . 7.1.).

From the y-ray intensity balance, i t seems reasonable to believe

that only the levels at 503-1 and 352.1 are fed in the ß -decay. The

Y~rays from the states <87.0 keV are probably highly co-.verted.

147 1487.3. Data obtained for the decay o f Ce and Ce

The decay of Ce and Ce show many s i m i l a r i t i e s such as having

approximately the same h a l f - l i f e (56 s and 48 s , respectively) and" a l -

most the same energy of the strongest Y~ray (269.1 and 269.7 keV,

respect ive ly) . I t is therefore sui table to discuss these nuclides to-

gether, as was also done in the papers [ I , IV, IX] .

The f i r s t h a l f - l i f e determination o f Ce and Ce was performed

by Hoffman e t a l . [Hof 64 ] , who obtained 65 ± 6 s and 43 ± 10 s , respec-147t i v e l y . Fasching [Fas 70] then determined the h a l f - l i v e s of Ce and

Ce to be 60 ± 3 and 48 ± 4 s , respect ive ly, by ind i rec t measurements.

These were the only data avai lable when we started our invest igat ion.

The f i r s t ' Ce Y"ray data was presented in papers [ I , I V ] , whi le

the f i r s t pa r t i a l decay schemes were published in [ I X ] . Some Y~ray data

has also been reported by Seyb [Sey 73] . Recently, the Grenoble group

obtained data from measurements on mass-sépara ted ' Ce samples.

102.

In the f i r s t SISAK paper [ I ] on the decay o f shor t - l i ved Ce isotopes,

no assignment o f Y";~ays to e i ther Ce or Ce was made because the

dif ference between the ha l f - l i ves was only 8 s , whi le the error l im i t s

in the Y" r a y h a l f - l i v e s were 3 - 6 s. However, the two strongest Y~ray

peaks at 269 and 292 keV showed ha l f - l i ves o f 58 ± 3 and 45 * 5 s ,

respect ively, and therefore they were assigned to Ce and Ce. In

the experir^nts preceeding paper [ I V ] , we performed v~Y coincidence

measureiw. s on a Ce f r ac t i on . A l l Y~rays being coincident wi th the 269

keV Y-ray were then assigned to

keV were assigned to Ce.

"»7,Ce, while those coincident wi th 292

In the Mainz experiments, where the measuring equipment yielded a

better energy resolution, we found that the 269 keV Y-ray had a FWHM-

rvalue that was approximately 0.6 keV greater than expected. A careful

computer resolut ion o f the peak showed that is was a doublet. The ener-

gies of the y-rays forming the doublet were 269-1 and 269-7 keV, re-

spect ively. As described in [ I X ] , i t was possible to connect the 269-7

keV component wi th the 291.8 keV y-ray s t i l l a t t r ibu ted to Ce be-

cause both y-rays were coincident wi th the weak 130.1 keV y- ray .

A consequence of the s p l i t t i n g of the 269 keV y-ray in to two peaks

belonging to d i f f e ren t nuclides was that many of the y - l i nes previously

a t t r ibu ted to Ce were assigned to Ce.

During th is work we learned of the results reported by Devi 11ers et

a l . [Dev 76] at the LOHENGRIN mass separator. These resuïts made us

feel confident about our own assignments. For some y- ray3, however, our

mass assignment is based on the assignments reported by Devi i le rs et

a l . [Dev 76] .

The ha l f - l i ves of Ce and Ce were measured both by the conven-

t ional technique and the TDD method. With the new assignments o f the

y- rays , the h a l f - l i v e s (calculated as a weighted mean value of the

ha l f - l i ves o f individual y-ray peaks) previously reported, 56.4 + 1.2 s

and 48.1 ,+ 1.1 s , have been changed to 56.7 + 2.3 s and 50.5 +_ 1.6 s.

These values are in agreement wi th those reported by Devi Hers et a l .

(55.0 + 3.0 s and 48.0 + 4.0 s , respect ively) .

From the coincidence dota obtained for the decay of Ce and Ce

(cf . [ I X ] ) , the decay schemes shown in f i g s . 7-4. and 7-5- are proposed.

The main d i f f i c u l t y was to decide whether the y-rays coincident wi th the

269 keV doublet were coincident wi th the 269.1 or the 269-7 keV y - ray .

This problem was solved via a channel by channel gating on 269 keV. I t

appeared that the 92.9, 439.8 and 832.2 keV y-rays belong to the 269-1

keV peak, while the 127.2 keV belongs to the 269-7 keV y-ray.

I t is not possible to draw any conclusions about nuclear propert ies

from these decay schemes. There i s , however, a s l i gh t relat ionship

between the level structure of Ce and Ce because both nuclei seem

to have several closely spaced levels between 90 and 140 keV.

103.

) o o <D «- CM * "o o» 'S o q o o o f - n o i a

r\

« ô « • n N o n M •-

a s - * 8 s s E« « « « o

a •* « • a& « r- r* •rt • «•> •- n

544 2

«67 3

3CZ O

289 S

92 9

O O147 Pr

Figure 7.<u Partial decay scheme of Ce. The scheme is based on the coincidences listed in

table 3, paper [IX].

332

723

3 7

62

0.7

422

0

325

0

36

93

347

227

3.8

374

4

onn 26

9 7

195

719

1 7

168

3

19S

710

5 2

74 6

121

211

6 8

g Oat

623 5

520 7

468 6464 8

195 8

121 21 1 6 89 8 59 0 4

1 4 8 P t

'5- P a r t i a l d e c aV « * « • °f "'Ce. The scheme is based on the coincidences listed intable 4, paper [IX].

10*» =

rl.h. Data obtained for the decay o f l<l9Ce and '50Ce

147 148Like Ce and Ce, the two most shor t - l i ved Ce isotopes studied by

\La ten

the SISAK technique, Ce and J Ce, have almost ident ical h a l f - l i v e s ,

5.7 s and '».I s , respect ively, and therefore the i r decay propert ies w i l l

be discussed together.

Before this invest iga t ion , no data had been published on the decay

of Ce or Ce. However, some results had been obtained by Seyb

[Sey 73] and Fischbach [Fis 7'»] by enploying f a s t , automatized o f f - l i n e

chemistry. Recently, T~ray energies belonging to the mass chains J49

and 150 were reported by Devi Hers e t a l . [Oev 76]. As w i l l be discussed

la te r , many of the y-rays belonging to Pr have been assigned to Ce

by th is group. These incorrect assignments are probably due to the s im i -

la r ha l f - l i ves of these nuclides (6.2 and 4.0 s , respectively) and the

lack of chemical separations. However, as for Ce and Ce, the com-

parison o f SISAK and LOHENGRIN data y i e l d a resul t that should be correct

wi th respect to both elements and mass.

The most intense short-1 ived y-rays present in the fast Ce f rac t ion

exhibi ted h a l f - l i v e s o f ~4 and ~6 s. These ha l f - l i ves were measured in

the organic phase, in which Nb, Br and I (mainly ~3 s ' ° V 7.1 s 101Nb,4.3 s Nb, 4.8 s ° Nb and 46 s ' 3 I) were also present. To obtain an

unambiguous element assignment, only those y~ r ays that were also iden-

t i f i e d in a str ipped water phase (cf . f i g . 3.8.) were a t t r ibu ted to Ce

(Nb remains completely in the organic phase). The ~4 s h a l f - l i f e could

be assigned to Ce from the growth-and-decay of Pr (cf . paper [ I I I ] ) .

F ig. 7-6. shows such a curve obtained by using the new SISAK 2 equip-

ment. The result is in good agreement wi th the previous data presented

in [III].

149The ~£ s h a l f - l i f e was, accordingly, a t t r ibu ted to Ce. This ha l f -149l i f e could not be connected wi th Pr v ia growth-and-decay curves, due

to the low y ray in tens i t ies o f that nuclide (cf. section 8 .4 .3 . ) .

149However, a problem was that only the strongest peaks of Ce and

Ce showed h a l f - l i f e wi th su f f i c i en t l y small er ror l im i t s to allow

mass assignment. Other y~rays could be connected to these y-rays via

T~T coincidence re lat ionships, but most of the y-rays have been assigned

according to the masses obtained by the LOHENGRIN group [Dev 76].

105c

£iaiire 7.6. Growth-and-decay of the 130.2 keV peal: of 15DPr measured in the Ce fraction.

Fig. 7.6. shows decay curves for the most prominent y-ray peaks ofCe and CS, respectively. Fig. 7.7. shows a growth-and-decay curve

tO6.

Figure 7.7. Decay curves of the strongest Y-ray peaks of Ce (57.9 keV) and '^BCe (109.5 keV).

robtained for the 130.2 keV y-ray of I 5°Pr. The ha l f - l i ves obtained, 5-7

and «t.1 s, are in agreement wi th the data of Seyb [Sey 733 (5-2 s and

3.9 s) and Fischbach [Fis lh] C».8 s and 3-7 s) whi le the LOHENGRIN

group obtained a shorter value for the 9Ce h a l f - l i f e (3-0 ± 0.5 s ) .

Their value fo r 1 5 0Ce, 5-9 * 0.3 s , is longer than our 4.1 ± 0.6 s ;

th is dif ference might be explained by the previously mentioned confu-

sion of 15°Ce and 1 5 0 P r y - r a y s , in the LOHENGRIN resul ts . Thus, i f the150Ce h a l f - l i f e was calculated as a mean value of the ha l f - l i ves mea-

sured for individual y-ray peaks, the p^aks belonging to 6.2 s pr

have influenced the h a l f - l i f e obtained.

To calculate reliable half-l ives for the weaker Ce y-ray peaks

has not been possible, since the spectra show too many closely spaced

peaks to allow the computer programs to perform a reliable peak area

determination.

ï i ray__da ta

The aquisition of good Y"ray data for Ce and Ce was d i f f i cu l t

due to the combination of low fission yields and low y-ray intensities.

Fig. 7.8., however, shows a y-ray spectrum of the fast Ce-fraction.

This spectrum was obtained from a decay measurement by subtracting the

third and fourth points (measured k - 8 s after start of measurement)

from the f i r s t point (measured 0 - 4 s). In this way, al l long-lived

149 150Ce s , „ 3 K i w c l r u m

Energy range O 1000 heV

1 fceV per channel

0» « oo t ;»>

500

CHANNEL NUMBER

Figure 7.8. T~ray spectrum of the most short-lived Ce isotopes. To obtain this spectrum, the

third and fourth spectra of a decay measurement (measured <i-8 s after start of the measurement)

were subtracted from the first two spectra (measured O-* s). The spectrum was measured in

an organic phase also containing Zr and Nb.

'07.

r\k9 150

Table ' . 2 . Energies of r-rays assigned to Ce and C*

T-r«r emrgl«». Ce Y - « y •*»••«)«», fcev

57.9

«6.6

101..6

129.2

1*5.2

380.1

864.5

893.0

I0J.)109.5

9~ISOCe(T. .«>10 - 15 s) activit ies were suppressed. The y-rays of

are listed in table 7.2.

149Our data for Ce are in agreement with the -esults of Seyb [Sey 73]

and Fischbach [Fis 74]. They are also consistent with the LOHENGRIN data.

Concerning Ce, our data is different from the LOHENGRIN results,

since the Grenoble group assigned the 469.0, 545.9, 720.6, 721.9, 804.4,

852.7, 931.5 and 1061.6 keVy-rays to Ce. From our Pr experiments

(cf. section 8 . 5 . ) , we are absolutely sure that these Y"lines belong to

Pr, since no other Ce activit ies have been observed in the Pr fraction

(except for the very strongest peaks of Ce). Many of the y-rays

also show coincidences with peaks known to belong to Pr.

108.

r8. DATA OBTAINED FOR THE DECAY OF NEUTRON-RICH Pr ISOTOPES

p1l|8Nd, 150Nd). This means that the levels of

The neutron-rich Pr isotopes are situated immediately "below" the

stable Nd nuclei ( I

the Nd nuclei, apart from Ce and Pr, are accessible not only by ß~-

decay studies of Pr, but also by nuclear reaction experiments on these

stable nuclei. Thus, the excited states of Nd, Nd and Nd have

been studied by Chapman et al.fCha 72] by employing the (t,p) Nd,146 148 148 150

Nd(t,p) Nd and Nd(t,p) Nd reactions and by Van der Baan

et a l . [Baa 75], who uti l ized the i£ |6Nd(d,d') | l | 6Nd, ]k&Hd(d,6') 1l|8Nd

and Nd(d.d') Nd reactions. The excited states of the even-odd|/i7 1/iQ 151

nuclei Nd, Nd and Nd have recently been carefully studied by

Roussi I le e t a l . [Rou 75, Pin 75, Rou 76a, Rou 76b, Pin 76, Pin 77a,

Pin 77b] by the reactions | / f6Nd(n,y) ' ^ N d , 1lt8Nd(n,y) l£>9Nd andl5ONd(n,y) 151Nd. Rousille et a l . also studied the B~-decay of 1/>7Pr

and Pr mass separated samples at LOHENGRIN (Grenoble) and JOSEF147(Julien). The level structure of Nd had ear l ier also been studied

by Wiedner et a l . [Wie 6?] by a (d,p) reaction on Nd.

Of the Pr isotopes with A > 146 only Pr had been carefully stu-

died before the SISAK investigation. This nuclide has a long ha l f - l i f e

(24 min) and a long-lived precursor (14 min Ce) which makes i t

easily accessible for chemical separations. Thus, Daniels at a!. [Dan 68]mi Iked

146Pr from a Ce fraction and performed measurements of y-rays,

T"T coincidences, ß-y coincidences and conversion electrons. The result

was a complete decay scheme for Pr.

] 4 7~ 149

The decay data on Pr available before this work were confined

to the half- l ives and a few y-rays. Recently, these nuclides have been

independently studied by the SISAK Collaboration (cf. papers [ I , IV,

V I I ] ) and Roussi I le e t a l . [Rou 76b]. Some y-rays of Pr have also

been observed by Ohyoshi et a l . [Ohy 73]-

In this section, the results obtained in the SISAK experiments on

Pr wi l l be cr i t ica l ly discussed and also compared to other decay

data and nuclear reaction data.

8 .1 . Data obtained for the decay of Pr

Although the delay time between the Ce extraction step and the Pr

back-extraction step (cf. f ig . 3.15.) was short in comparison with the

ha l f - l i f e of Ce, the amount of grown-in Pr allowed a check of the

103.

rdata published by Daniels et a l . [Dan 68] . The results obtained were

consistent with their data. The isomeric state suggested by Hoffman

and Mi ehe I sen [Hof 651 was not observed.

1478.2. Data obtained for the decay of yPr.

In 1964, Hoffman and Daniels [Kof 64] separated a 12 min Pr activity147

that was assigned to Pr. The Pr was isolated from a chemically se-

parated Ce fraction. To substantiate their assignments of this activity

to mass 147,they also performed (Y»P)~ react ions on enriched Nd. In1471970, Fasching [Fas 70] made an investigation of Pr T-rays. The de-147cay of Pr was not studied again until recently, when results were

published by the SISAK group [ V I M ] , Dorikens and Doriker^ - Vanpraet

[Dor 75] and Roussi He et a l . [Rou 76b, Pin 75].

The assignment of the 12 min Pr activity to A = 147 has been ver i -

fied by many independent methods, in addition to the ( Y , P ) experiments147performed by Hoffman and Daniels [Hof 64 ] , Pr has been studied at147mass separators. Furthermore, the decay of Pr populates most of the

levels found in nuclear reaction experiments l ike Nd(n,y) Nd.

The h a l f - l i f e obtained in the SISAK experiments, 12.0 ± 0 . 2 min is

in good agreement with the values reported by other groups [Hof 64,

Fas 70].

110.

8i2i2;_ÏII§Y_data_and_deçay_sçheme

146-149A Y-ray singles spectrum of the Pr fraction is shown in f ig .

8 .1 . This spectrum was measured on an cation exchange column is front

of the detector (cf. section 3 .5 . ) . I t is dominated by the intense148301.8 keV Y-ray from Pr; the high sample strength did, however, allow

good Y~Y coincidence measurements not only for

and ' « P P .

but also for 1*7 D

Table 8 .1 . shows the coincidences obtained for 'Pr and f ig . 8.2.

shows the corresponding decay scheme. When comparing the decay scheme

deduced from our data with the level scheme proposed by Wiedner et a l .

[Wie 67] , i t was obvious that the level energies obtained by that group

— -<S 2"I "D- • m3 n

d3

COUNTS PER CHANNEL

1023.2 Pc -148

1248 S Pi-148

1357 9 Pt 148 o

ISi

COUNTS PER CHANNEL

P.

517.0 P i . 147, Pr -MS

• 554.7 Pr-147

— — 677 9 Pr -147

£ «18 4 Pi-14«

I — 838.8 Pi-1498414 Pi-147

- • 8 0 3 Pi -148

897.8 Pi -148

.-721.5 Pi-148' -724.3 Ci -148

825 S Pi -148

- 903 5 Pi • 148

; -947 3 Pi-148

-998.0 Pi-147'-J 1 1 1 1 1 L_

S 3

I I

COUNTS PER CHANNEL

77 S Pi.147I t S Pi-147

" " " 11.1 Pi-14»108.8 P i l C I

-119.7 Pi-149127» Pi. 147

138 4 Pi-t49. '188.0 Pi • 149-182.4 Pi -149

-185.0 Pi -149-192.8 Pi -149

•-188.7 Pi-147- -212.5 Pi-149

-214 6 Pi.147224 2 Pi -149

- 227 3 Pi -149- 239.9 Pi • 149-247 0 Pi • 148'2480 Pi. 147

-•258 2 Pi-1492948 Pi -149. C« • 149

I "JluTTr. 147- -3171 C l U I

- 329 8 Pi.147•3321 Pi149-335.7 P, 147

- 388 9 . 388 3 Pi -149

3891 Pi. 147401 3 Pi.149413 7 P|.|47432 8 Pi-149450.8 Pi-148

487.0 Pi-14 7477.9 Pi-147

3018 Pi. 14* I

- I -033-J

i!

Table 9.1. Energies, relative intensities and coincidences observed for y-rays assigned to Pr.

(keV) Observed coincidence«

49.9 21-5 77.9(s). B6.5U), 186.71s). 328.81s). J3S.7(s). 389.1 (s), 4I3.7U1. 554.7(«),577.9(s), 641.4(s). H36.5U). 1300.4U)

77.9 63.6 86.5(s), I66.7(s), 249.a(s), 328.8<n). 335.7(s>. 389.Ks). 55<t.7<s), 577.9(s),641.4(5). 1136.5(5)

86.5 86.7 77.9(s). I27.9(s), 249.0(s), 328.8M. 5S4.7U). 577.9<s>.127.9 42.9 ee.5(s). 186.7<s), 249.0(s), 328.8U), 33S.7(s), 369-Ks), 554.7U). 577.9U).

641.4(s). 1136.5(s), 1183.0(s)186.7 5.4 49.9<s). 77.9(s), 127.9(s). 477.9U)214.5 7.6 554.7(5), 577.9(s)249.0 6.9 None314.7 100.0 477.9(s). 627.5<s), 996.0(s), 10B3.5U)328.8 20.0 49.9(s), 77.9(s), 127.9(s), 249.0(s), 335.7(s), 413.7(m)335.7 25.1 49.9(s), 77.9(s). 127.9(s), 328.8(s)389.1 10.3 77.9(5), 127.9(s)413.7 5.3 49.9W467.0 8.9 49.9(m)477.9 26.5 127.9(s), 186.7(s), 314.7(s)517.0 3.0 None554.7 33.8 49.9(s), 77.9(s). 86.5(s), I27.9(s), 214.5(s)577.9 7B.3 49.9(s), 77.9(s), 86.5(s). 127.9(s), 214.5(5)627.5 <1 Not gated641.4 84.4 49.9(s), 77.9(s), 127.9(s)942.2 5.8 None996.0 8.5 314.7I5)1083.5 - Not gated1136.5 7.9 49.9(.,j. 77.5W. 127.91")1163.0 5.1 127. 'jM1264.3 7.9 'io-'e1300.4 - Not gated1310.8 - Not gated

The uncertainty in the Y~ray enerqies is estimated to be ±0.5 keV. The uncertainty in the T~rayintensities is estimated to be ±)0% for strong peaks {I >10%) and ±20% for the weak ones.

ii,7were 50 keV too low. This means tnat the ground state o f Nd was

not excited in the (d,p)-react ion employed by Wiedner et a) . This

f a i l u re Of the one-nucleon transfer experiment can have two explana-1/4.7

t ions: e i ther the ground state of Nd is co l lec t ive or i t has a

high sen ior i ty (many unpaired neutrons). I f so, the (d,p)-react ion is

unsatisfactory for exc i t ing the ground s ta te .

The par t i a l decay scheme proposed by Hoffman and Daniels [Hof 64]

disagrees with the present scheme not only wi th respect to the ground

state ( th is group also considered the kS.S keV state as the ground

state) but also to a 0.61 - 0.335 - 0.105 - 0.127 - O.O78 MeVy-ray

cascade that was not observed by us.

Compared to the decay schemes proposed by Dorikens and Dorikens -

Vanpraet [Dor 75 ] , Pinston et a l . [P i n 75] and Roussi I l e et al.[Rou 76b],

there is a disagreement concerning the 1261 keVy . 'ay which was by then

considered to be coincident wi th the 49.9 keVy-ray. We have not ob-

served such a coincidence. This absence of a coincidence i s , in f ac t ,

supported by the decay scheme of ref. [Dor 75] , in which the kS - 1261

keV cascade is not tagged with any "coincidence dot" . Probably, the

1261 keV l ine was placed there only because i t f i t s well into the gap

between the 1310 and k3 keV levels.

112.

rT^2'12min \

O« • 2 7 M.V \' I Hot 641*

isi

« o ID m

s ss st « A 0), . «o r» r*.* <M r> r-« n « u>

o m n

S^ - ° °

S 5

ID m «> -n « S *

1350 3

1310 8

1264 3

792 6769 3

147Nd

517 0

4(136

314 7

214 5

127 9

49 9

00

13107

12643

7926

7693

5167

2149

5/2. 7/2

5/2". 7/2"

5/2". 7/2"

3/2

3/2"

3/2"

3/2"

1/2-

S/2"

7/2"

B/2-

Fiqure 8 . 2 . Par t ia l decay scheme of ' 7Pr. The scheme is based on the coincidences l is ted in

table 8 . 1 . As a comparison, the levels and spins reported by Roussille [Rou 76b] are indicated.

The level schemes of Roussi He et a l . [Rou 75, Rou ?6a] and Dori-

kens and Dorikens-Vanpraet [Dor 75] contain more y - l ines than the pre-

sent scheme. This is mainly due to the method employed. When producingU 7 P r by (n,Y) reaction on enriched 1/|6Nd (Pinston et a l . ) or (y.p)

reaction on enriched Nd (Dorikens and Dorikens-Vanpraet) there

w i l l be almost no other nuclides than Pr present. Furthermore, i t

is not necessary that a l l these levels are populated in the ß -dtcay.

The decay scheme proposed by Pinston e t a l . [Pin 75] , which is based

on measurements on mass separated samples from the LOHENGRIN separator,

shows essent ia l ly the same levels and y-rays as our scheme. The two

highest s ta tes, 1350.3 and 1398.2 keV were, however, not reported by

Pinston et a l .

147 -

The spin o f the ground state o f Nd has been measured as 5/2

[Led 67] by the atomic beam method. Fo- Pr, the spin 5/2 has been

suggested as the most probable [Rou 76b]. Roussi l i e ' s thesis [Rou 76b]l*»7also contains a penetrat ing discussion of possible spins of a l l Nd

levels. These spins are indicated in f i g . 8.2.

8.3. Data obtained fo r the decay of Pr

Like 7pr, Pr was f i r s t i den t i f i ed by Hoffman and Daniels [Hof 6*»].

The h a l f - l i f e was determined as 2.2 min. Later on, Ohyoshi e t al.[Ohy

73] reported on the h a l f - l i f e and some y-ray energies o f Pr. The

h a l f - l i f e obtained, 2.1 min, agreed w i th the value o f Hoffman and

Daniels. The next invest igat ion was performed by the SISAK group [ IV ,

V I I ] . In th is inves t iga t ion , we a t t r ibu ted several new Y~rays to Pr

and we also proposed a pa r t i a l decay scheme. Recently, the Pr decay

has also been studied by the Grenoble group [Dev 76 ] , who reported on

y-ray energies and i n tens i t i es .

In addit ion to the decay data, valuable data on the Nd levels

has been contr ibuted by Chapman et a l . [Cha 72] and Van der Baan et a l .

[Baa 75] using nuclear reactions.

8 i2 ii i_Mass_assi.gnment

The reason that we assigned the y-rays l i s ted in table 8.2. to the decay

of Pr is mainly that most o f them showed strong and unambiguons coin-

cidences wi th the 301.8 keV y-ray known to be the 2 •+ 0+ t rans i t ion in

Nd. The levels in the decay scheme deduced by the author also agreed

well wi th the levels found by Chapman et a l . and Van der Baan et a l .

Recently, the Pr Y-rays were also iden t i f i ed et the LOHENGRIN mass

separator [Dev 76] .

The h a l f - l i f e obtained from the SISAK experiments, 2.2 ± 0.1 min,

is in good agreement wi th other published data [Hof 6k, Ohy 73] ,

rThe Y"ray spectrum of Pr (cf. f i g . 8.1.) is dominated by the 301-8

keV peak o r ig ina t i ng from the 2 • 0 t rans i t ion in Nd. No other peak

i s , in fac t , stronger than ~l I % re la t ive to the 301.8 keVY" l 'ne but

i t was no problem to obtain good coincidence data for Pr. To give

an impression of the qua l i ty of the coincidence spectra, f i g . 8.3. shows

the gate at 301.8 keV.

148The coincidences obtained from the decay of Pr are l i s ted in table

8.2. together wi th the re la t ive in tens i t ies of the y r a y s . From these

coincidences, the decay scheme shown in f i g . 8.4. has been deduced. The

spins and par i t i es indicated in the scheme have been taken from re fs .

[Cha 72] and iBaa 75].

The f i r s t excited state at 301.8 keV should be the one-phonon v ib -

rat ional state belonging to the ground state rotat ional band. Thus

the spin and par i ty of the 301.8 keV state is 2+ . According to nuclear

level systematics (cf . f i g . 8 .5 . ) , the level at 752.4 keV should be

the corresponding 4 s ta te . This is slrongly supported by the absence

of any crossover to the ground s ta te .

At 917-2 keV we found a level which might be the 0+ level found by

Chapmar. et a l . [Cha 72] at 911 keV. The energy d i f ference, ~6 keV,

seems reasonable since most of the other ( t ,p ) - leve ls have energies

Table 8.2. Energies, relative intensities and coincidences observed for Y-rays assigned

148.0171.5247.0301.8

450.6615.4636.8660.3697.8721.5825.5869.4903.5947.3

1023.21157.41171.21248.81357.913B1.61419.71521.521332635

<l< l0.7

100.0

5.35.42.53.6

10.98.33.17.53.43.49 .02 .01.36.6

10.44.72 . 10 . 9

1023.21697.8(s]301 .8(5) .148.01m),721 .5 (s ) ,1381.6(s>247 .0 (s ) ,301.81s),301 .8 (s ) .247.0(m),171.51s .301 .8 (s ) .Î O l . B ( s ) ,301.8(s) ,301.8(s301.8(5148.0(m301.8( i903.S(m825.5(s301.8(s301.81s301.8(5)301.8(s)301.8(5)301.8[s

450.6(s)171.5(m)825.5(m],

, 1419.71»301.81s)1157. Mm)721.5(5) ,301.8(s) ,301.8(s) ,636.8(s)947.3(m),9 0 3 . 5 ( 08 6 9 . 4 ( 5 ) ,825 .51»)636.8(m)615.4(m)

2 4 7 . 0 ( s ) , 4 5 0 . 6 ( s ) , 6 1 5 . 4 1 s ) , 6 3 6 . 8 ( 5 ) , 6 6 0 . 3 ( s ) , 697.81s8 6 9 . 4 ( s ) . 9 0 3 . 5 ( s ) . 9 4 7 . 3 1 s ) , 1 1 5 7 . 4 ( m ) . 1 3 5 7 . 9 ( s ) .

1 , 1521 .5 (m) , 2 1 3 3 1 s ) , 2635 (s )

1023.2(m|6 9 7 . 8 ( 5 )6 6 0 . 3 ( s )

1248 .8 (s )

1171.2(m)

115,

Gat« 301 S k»V

Compton cortaction subtr«ci*d

Smooihad

1 fcaV par chinnal

OlB O

SE s

^^^^^^^

B1 S S

1000

| | s

hl*1000

CHANNEL NUMBER

Figure 8.3. Coincidence spectrum showing the T-rays in coincidence with the 301.8 keV peak.

3 - 6 keV lower than the energies from the present decay measurements.

The assumed 0+ state which was not excited in the (d,d')-experiments

performed by Van der Baan e t a l . [Baa 75] , is de-excited via a 615.4

keVy-ray to the 2 s ta te .

The level at 999.6 keV should be the 3~ state [Baa 75]. I t is de-

excited via a weak t rans i t ion to the 4 state and a strong one to the

2+ l e v e l . l t is also connected to the second 2+ state at 1171.2 keV by

a weak Y~ray at 171-5 keV.

The next leve l , 1023.2 keV, is assumed to be the 1 state connected

to the 3 state [Baa 75]. This assignment is based on level systematics

for the neighbouring N = 88 nuc le i , (cf. f i g . 8 .5 - ) .

The second 2+ level is found at 1171.2 keV, according to the assign-

ments of Van der Baan et a l . [Baa 75]. This assignment is supported by

the existence of y-rays to the 0+ and 2+ states (1171.2 and 869.4 keV,

respect ively) .

The level at 1249.0 keV is probably ident ical wi th the 1241 keV

level reported by Van der Baan et a l . [Baa 75]. This level was f i r s t

assigned a spin 2 , 3 or 5 ; from the (d,d')-experiments 2 seems to

be most probable [Baa 75]- This assignment is fur ther supported by the

de-excitat ion of the 1249.0 keV state via y-rays to the 0+ state (1248.3

keV) and the f i r s t 2+ state (947.3 keV) . The energy difference between

the t rans i t ion d i rec t l y to the ground state (1248.3 keV) and the sum of

the cascade y-rays (1249.1 keV) is here somewhat greater than usual but

r" • P r \

T1/2-2 2min \

aP '4SM«V \I Hot 6« I l>

Blral>.%

1312

24139 0»

100

«CM

t<

<<

(

o°«0

5m

a3D*

247

0

(0 7

)69

7 8

(

10 9

)72

1 5

(

8 3

)

0

nM

171

5

(<

1)

•86

94

(76

)

—

1248

8

I 6

6)

660

3(3

6)

1367

9(1

0 4

)

9 7

(21

)

82

SS

(3

1)

903

S (

34

)1

15

74

(20

)

o

Z937

2074 S

1822 3

1721 S1683 41659 9

1249 0

11712

1023 29996

752 4

3018

22572200

20932035

18771813

1724 3"1883 4«

1241 (2\3-

1169 2*

'Si? V911 0*

302 2*

Figure 8.fr. Partial decay scheme of Pr. The scheme is based on the coincidences listed in

table 8.2. As a comparison, the levels and spins obtained by Van der Baan et al. [Baa 75} are

shown to the right. The spins adopted by us are shown to the left.

s t i l l w i th in acceptable l i m i t s . The energy f i n a l l y a t t r ibu ted to the

level is a weighted mean value.

The most in terest ing levels in the present decay scheme are t.ie 0+

state at 917.2 keV and the 2+ state at 1171.2 keV. The l a t t e r is tie-

excited via Y-rays to the i " (1023.2 keV) and 3~ (999.6 keV) members

of the K = 0 octupole band. This decay pattern resembles the one ob-

" 7 .

ISO«

Figure 8.5. Level systematics for the Z - 60 and N = 88 nuclei. The data have been taken from

the present work and refs.lHuc 73, Lie 7M.

tained by Wi rth et a l . [Wir 75] for the de-excitation of the 2+ level

at 1292.8 keV in 52Sm populated in the decay of 4.2 min 152Pm. This

level is related to a 0+ state at IO83.O keV.

In ref. [Wir 75]. the 0 and 2 states were interpreted in terms1S2

of nuclear shape coexistence, i.e. the nucleus Sm is less deformed

in these excited states than in the ground state. On this basis i t

might be suggested that the two levels at 917.2 and 1171.4 keV in Nd

should be interpreted in a similar manner. From the present data,

however, i t is not possible to draw any definite conclusions about this

possible nuclear shape coexistence in Nd.

The ß-feeding of the Nd levels has been calculated assuming a

negligible feeding of the ground state. The results are indicated in

f i g . 8.4. Unfortunately, i t is not possible to conclude anything about

the val idity of this assumption. Thus, in Nd, approximately 40 Z

118.

of the (J-decays populate the ground state, so there is actually no

reason to believe that the Pr decay is completely dif ferent. The

calculated ß-feeding gives, however, an indication of which states

3re populated.

The strongest ß-branches are those feeding the 2 state at 301.8

keV (44 % relative ß-intensi t y ) , the l " state at 1023.2 keV and the

state at 1683.4 keV. The 2+ state at 1171.2 keV is probably not (or

very weakly) fed in the B-decay. The other levels seem to be almost

equally populated.

The ground state spin of Pr has been computed by Seeger [See 70]

to be either 0+ or 3+. With respect to the ß-feeding of the Nd

levels, the most probable of these spins is 3 because the spin 0

would require a second forbidden transition to the 0 state at 917.2

keV. I f so, the ß-feeding of that level should be less.

1498.4. Data obtained for the decay of Pr.

The f i rst research group that reported on decay data for Pr was

Hoffman and Daniels [Hof 64], who found a weak ~2.3 min act iv i ty after

irradiation of Ndwith bremsst rah lung. The results of Hoffman and

Daniels were then confirmed by Van Klinken and Taff [Kl i 67] in a s i -

milar experiment. Their results also included energies of some strong

y-rays, the total ß-decay energy and a simple decay scheme. In a study

of fission products by a fast electromigration technique, Ohyoshi et149

a). [Ohy 73] determined the ha l f - l i f e of the two strongest Pr y-

rays to be 2.9 * 0.1 min.

The f i r s t detailed investigations of Pr were performed indepen-

dently by the SISAK group [IV, VII] and the group working at the LOHEN-

GRIN mass separator [Pin 76a, Pin 76b, Rou 76b]. These investigations

yielded several new y-rays and also decay schemes.

149Nuclear reaction data on Nd has been reported by Pinston et a l .

[Pin 76a] and Burke et a l . [Bur 73].

149.,The main reason for our assignment of the ~2.9 min y-rays to '"'"'Pr149

is the f i t between the Nd level scheme obtained in our decay experi-

ments and the level schemes deduced from nuclear reaction data. Further-

119.

rmore, in the beginning of the Pr experiments we performed runs in which

a delay tube, long enough to allow a complete decay o f Ce, was i n -

serted before the Ce extract ion step (c f . f i g . 3-15.) . In this way, we

were able to discriminate between y rays belonging to 2.2 min Pr and

2.9 min "*9Pr.

Recently, the assignment of the 2.9 min T-rays to JPr was confirmed

by the results obtained by Roussi l i e [Rou 76b] in measurements on mass

separated samples.

SiiJiii.Haif-life

The SISAK experiments performed on Pr included ha l f - l i fe deter-

minations made with the conventional technique on samples collected on

a cation exchanger. The value obtained, 2.9 ± 0.3 min, is a weighted

mean value of the half- l ives of the strongest peaks. I t is in agreement

with the value published by Ohyoshi et a l . [Ohy 73], while i t is longer

than the ha l f - l i f e reported by Roussi l ie [Rou 76b] (2.1 ± 0.2 min) and

Van Klinken and Taff [Kl i 67] (2.3 min).

Some experiments were also specially designed to confirm or rejec«-

the isomeric state suggested in [See 74], These experiments (cf. the

discussion in section 8.4.3.) clearly show that there is no short-

lived isomeric state in Pr.

8.4.3.. f r a y . data_and_deçay__sçheme

147-149As demonstrated by the y-ray spectrum o f Pr shown in f i g .

149

8 . 1 . , the intensity of the Pry- l ines is rather low. This in-

fluenced the quality of the coincidence spectra negatively. Neverthe-

less i t was possible to obtain the coincidence data shown in table 8.3.

The decay scheme derived from these data is shown in f i g . 8.6. This

scheme is a somewhat extended version of the scheme published earl ier

by us [V I I ] ; the addition of some y-rays results from a careful re-

investigation of a l l available Pr data (recorded in Oslo and Mainz).

The levels found are in agreement with the decay scheme published

by Roussi l ie [Rou 76b] except for the levels at 459-7, 482.8, 548.5,

705-1 • 706.7 and 741.5 keV which were not confirmed. The number ofy-rays f i t tod into the present scheme is less than in the scheme of

149Roussi He; this is because of the low Pr y-ray intensities (45 %

120.

rTable 8.3. Energies, relative intensities and coincidences observed for Y-rays assigned to P

(keV) Observed coincidences

93.1108.5112.2119.7120.5138.4156.0162.4165.0182.6207.6212.S220.7224.2227.3238.6258.2313.1316321.0332.8365.9366.3406.3408.5432.6S17.0623.0662.4675.7716.2742.8755.6782.2

30.686.0<5

42.642.6

100.017.034.193.910.119.9<55.3

12.414.011.944.4

53.235.735.733.16.9

41.3

9.410.65.9<58.35.17.7

) . 224.2{m), 366.3U), 408.5M165.01™)112.2(„), I62.4(s), 207.6(9), 212.5108.5<r»>138.4(H)165.0(m)I19.7(m), 182.6<s), 227.3(m), 432 .6 (s ) , 675.7(m), 742.8(ni), 782.2165.0(s)108.5<s)93.K»), 156.0U), 238.6W, 406.3(s), 7l6.2<w), 755.6M138.4U)108.5(s)108.5(w)None108.5(w)138.4(s)165.0(s)623.0(mj, 662.4MNot gaudNot gatedNol gatedNoneNone108.5M165.0(s)I08.5M138.4U)Not gated258.2(m)258.2 (m)13B.4WNot gated138.4(m)165.OM138.4(„)

The uncertainty in the Y-ray energies is estimated to ±0.5 keV and the uncertainty inthe Y-ray intensities to ±20î.

of the 3-decays feed the levels 5I65.O keV). ?Z*9Pr is a good example

of the fact that nuclides close to a stable (or long-lived) nucleus

in the ß-decay chain are favoured by mass separator studies, since the

precursors can be allowed to decay completely before the measurement

is started. The (n ,y)-experiments performed by Pinston et a l . [Pin 76]

permitted an easier construction of this scheme.

The spins and par i t ies, according to Roussi l ie [Rou 76b] are indi-

cated in f ip . 8.6.

Generally, the y-transi tions agree well but there are some excep-

tions. Thus, the 119.7 keV transition seen in the SISAK experiments

shows a coincidence with the 138.*» keVy-ray, while Roussille consi-

dered i t coincident both with 138.4 and 165.0 keV. The y-ray intensi-

ties do aîso di f fer for some Y-rays. The difference is quite small

except for the 517.4 keV y-ray which was assigned only to I / f7Pr in the

SISAK investigation.

There have been some suggestions of an isomeric state in ' ^P r .

121.

• 2J minMaV \I Hof 641\

Qp-30MaV \41\

2« 2 * • •"_ - ^ o* «o «^ • » «

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61560 (

17

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182

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212.

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1

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7;23

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D "1 c• - 1

D)0

517

0

1|

1

11

1

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406 3

(

33 1

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32 6

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920 6

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571 3

517 0

474 8

403 6

365 9332 832103161285 5270 92582

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0 0

920 7 Î5/2*)

8B1 4 5/2 *

8143 (1I2'.3I2'J

709 77051

57155485

3/2"5/2*

3/2-(3/2-)

517 5 (5/2.7,2)

4828474 6459 7

4038

365 9

332 9321 1316 4285427082583

220 7

1651138 5

108 5

5/2-: 7/2"

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3/2"

(T/T-Î1/2". 3/2"

9/2*3/2"

9/2"

3/2"5/2"

7/2"

figure 8.6. Partial decay scheme of * W . The scheme is based on the coincidences listed in

table 8.3. As a comparison, the levels and spins reported by Roussi l ie [ftou 76b] are indicated.

Thus, a ~28 s a c t i v i t y w i th y r a y s at 70 and 138 keV has been reported

[See 7k]. The present author's impression is that the 70 keV l ine is

the one known to belong to 25 s La (cf . section 6 . 3 . ) . because this

Y ' l i n e has never been observed by us in any chemically separated Pr

f r ac t i on . Furthermore, in the experiment we performed at the OSIRIS

mass separator ( to establ ish our assignment of some y rays to La),

the 70 keV l ine could be unambiguously a t t r ibu ted to mass 145-

122.

1 9In the present invest igat ion, the decay curves obtained for Pr

y-rays in the chemically separated Pr f ract ion have not shown any

component wi th a h a l f - l i f e shorter than ~2.5 min, although some mea-

surements were designed for ha l f - l i ves between 5 and 60 seconds. There149

remained, however, a poss ib i l i t y that a Pr isomer is formed only

d i rec t l y in the f i ss ion process. I f so, the isomer would not be pre-

sent in the Pr f r a c t i o n , since i t is based on a Ce extract ion followed

by a back-extraction o f grown-in Pr. The isomer should, however, be

present in the La f r a c t i o n , since a l l t r i va len t lanthanides are present

in that system (cf. section 3 .6 . ) . Measurements showed that the only

Pr Y~ray energy indicat ing a shorter h a l f - l i f e was 165 keV. A check

of the coincidence data led us to assign th is y-ray to the decay of

La. The experiment at OSIRIS recently ve r i f i ed th is assignment.

The conclusion is therefore that there should be no isomeric state in1 J(9

Pr. This result is in agreement wi th the data published by Roussi l i e[Rou 76b].

8.5. Data obtained for the decay of Pr.

The only decay data available on Pr when the SISAK investigation

started were the results published by Ward et al. [War 70]. This group150.irradiated enriched Nd with fast neutrons to obtain the (n.p)-product

he F150,

Pr. The peak found at 131 keV, corresponding to the 2 -* 0 t r ans i -

t ion in "•* Nd, decayed wi th a 6.1 ± 0.3 s h a l f - l i f e . Ward et a l . also

determined the Q -value as 5.7 ± 0.3 MeV.

SISAK data on 50Pr has been published three times. In ref . [ I , I I I ] ,

we reported on the h a l f - l i f e while ref. [V I I ] includes coincidence data

and a pa r t i a l decay scheme.

The decay of ' Pr has recently also been studied at the LOHENGRIN

mass separator [Dev 76]. The results obtained there are, however, not

very conclusive since most Pr y-rays were assigned to Ce.

Like the other stable Nd nuc le i , Nd has been studied by means of

( t ,p) and (d.d1) reactions [Cha 72, Baa 75]. Thus the levels of '^°Nd

are wel l establ ished.

8.5.1._Mass assignments

The reason why we, l i ke Ward et e l . , a t t r ibu ted the 6.2 s ac t i v i t y

to Pr is that the energy o f the y-ray peak agrees with the energy

123.

rof the 2 + -»• 0+ t rans i t ion in Pr. Furthermore, the level scheme ob-

tained from the coincidences wi th th is peak is in exel lent agreement

wi th the Nd levels obtained in nuclear reactions experiments.

The h a l f - l i f e of '^°Pr was f i r s t determined to be 9-1 ± 0.*» s f l ]

in a La- f rac t ion . Later measurements have proven that th is value was

too high because the las t point in the measurement occurred already

at a delay time of 22 s (the measurements were carr ied out using the

TOO technique [ I I , Aro 7'*]). Thus, the curve was s t i l l being influenced

by the decay of Ce. By careful examination of the decay curve in

ref . [ I ] , a s l i gh t curvature orginat ing from the growth-and-decay can

be observed.

The next Pr h a l f - l i f e determination was performed on a chemically

separated Pr f rac t ion in order to minimize the influence from Ce.

The value obtained, 6.2 ± 0.2 s , was in good agreement wi th the values

reported by Ward et a l . [War 70] and [Sey 73].

Recently, the decay of the 130.2 k e V y r a y peak was studied by means

of the SISAK 2 system, which allows separation of stronger samples (the

transport times are shor ter ) . The measurements were performed on a Ce-

f rac t ion so that the h a l f - l i f e of Ce could also be obtained from

the growth-and-decay curve. The results J».l s ( Ce) and 6.2 s ( Pr)

were in good agreement wi th ea r l i e r measurements [ I I I ] . Fig. 7-7. shows

the growth-and-decay curve obtained in th is experiment.

Pr is so far the only nuclide on which we have performed Y~Y

coincidence measurements using the SISAK 2 system. Fig. 8.7. shows a

y r a y singles spectrum of the Pr f ract ion obtained by th is system.

The Pr peaks are strong compared to the peaks of long- l ived Pr iso-

topes. There i s , however, a rather heavy contamination wi th ^ I in

this f rac t i on . This contamination caused no great d i f f i c u l t i e s in the

evaluat ion, since the decay character ist ics of I are well known [Ert 71]

The y-rays a t t r ibu ted to the decay of ' Pr are l i s ted in table

8.k. together wi th the i r re la t ive in tens i t ies and coincidences. The

decay scheme deduced from these data is shown in f i g . 8.8.

rEnergy rang* 0 2000 fc»V

1 k«V p»i charmai

1000CHANNEL NUMBER

Figure 8.7.T-rav spectrum of the Pr fraction. The spectrum was recorded on-line during 30 minutes.

Al l levels found in the SISAK investigat ions agree with the levels

a t t r ibu ted to 5 Nd by Van der Baan et a ) . [Baa 75]. However, the 6+

state at 721 keV suggested by Van der Baan et a l . is probably not s i g -

n i f i c a n t l y populated in the decay of ' Pr since we have not been able

to see the corresponding 3^0 keVy-ray to the k+ s ta te .

According to level systematics for the neighbouring Z = 60 and N =

90 nuclei (cf . f i g . 8 . 9 . ) , the levels at 130.2 and 38l.it keV should

be the 2 and k members o f the ground state rotat ional band.

The levels at 676.1 and 850.8 keV are presumably the 0+ and 2+ states

belonging to the K = 0 ß-vibrat ional band [Baa 75]. Van der Baan et a l .

predicted that the level they found at ~8k8 keV was an unresolved

doublet consist ing of a 2 and a 1 s ta te . In the present invest iga-

t i on , this level has s p l i t up into two levels at 850.8 and 852.7 keV.

The reason why we assigned spin 2+ to the 850.8 keV state and 1~ to the

state at 852.7 keV is the de-exci tat ion pattern of these levels. Thus

Table 8.4. Energies, relative intensities and coincidences observed for Y-rays assigned to '5 0Pr.

Observed coincidences

130.2251.2WS545.9553.3720.6722.llBOM852.7931.5

1061.6

100.013.«

62.725.5IS.I12.4

251 Its), 469(m), 545 .9(s ) . 553.3(»), 720 .6(s ) , 722 .Ms) , 804.4(5) , 931.51m)130.2(s) . 5 5 3 . J H ,13O.2(«n), 251.2{m)130.2(s)NOL gated130.2(s)130.2(si130.2(m)None130.21m)

Th« uncertainty 'n the v a y energies is estimated to be ±0.5 fceV. The uncertaintensi t ies is estimated to be ±20%.

I Wir 70!

0)

SS

(134

)25

1 2

9

3

n

»

•

§ i

•

n;

ISS

ZI

N*N

Ü04

4

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853

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106

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852 7850 8

6761

1062

931

851

721

677

2*

3«

2 M

6»

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382 4*

re 8.B.Partial decay scheme of Pr. The scheme is based on the coincidences listed in

table 8.4. As a comparison, the levels and spins obtained by Van der Baan et al.[Baa 75] are

ind'cated» as well as the spins adopted by us.

the assumed 1 level at 852.7 keV decays via y-rays to the f i r s t 0+

and 2 states. This is analogous to the de-excitat ion of the 1 state

in Nd. The 469 keV t rans i t ion from the 850.8 keV state to the k+

state also makes the spin assignment of 2+ (ûl = 2) to th is state more

probably than l " (AI = 3 ) .

Level systematics ( f i g . 8.10.) and ref. [Baa 75] indicate that the

level at 35*t.6 keV is probably the 3 s tate. I t is de-excited via y-

rays to the f i r s t 0+ and 2 states.

F ina l l y , the level found at 1061.7 keV corresponds to the 1057 keV

level reported by Van der Baan et a l . [Baa 75]. I t is assumed to be the

126.

r

2-

If—12-1(y)

4"

2-

»11

122

1B«Gd 156Dy

Figure 8.9. Level systematics for the Z = 60 and N = 90 nuclei. The data have been taken fron

the present work and refs.[Bow 70, Nuc 73].

band head of the y v i b r a t i o n , which makes i t a 2 s tate.

I t is not possible to discuss the feeding of the Nd levels in the

decay of Pr, because the calculated Y-ray in tens i t ies are incomplete.

Thus, there was no poss ib i l i t y to compute the in tens i t ies of the 720.6

and 722.1» kev t rans i t i ons , since these y-rays from a mul t ip le t together

with the 721.5 keV peak from 1 / |8Pr and the 72A.2 keV y-ray from the145small amount of Ce present in the Pr f rac t ion . An ocular inspection

of the mul t ip le t indicates that the 720.6 keV peak has a higher inten-

s i t y than the 722.4 keV peak. This is consistent with the spin assign-

ments.

Seeger [See 70] estimated the ground state of Pr to be 0 or 3 •

There is no point in t ry ing to conclude from the present data which spin

is most probable.

127.

r9. FIGURE INVESTIGATIONS OF THE HEAVY La, Ce and Pr ISOTOPES

The measurements performed so far include h a l f - l i f e determinations,

y-ray singles measurements and Y"Y coincidence measurements. To permit

more detai led conclusions about the nuclear structure of the l igh t

lanthanide elements, i t is also important to perform ß-radiat ion mea-

surements and Y~Y angular corre lat ion measurements. Such measurements

w i l l be possible in 1-2 years.

Angular cor re la t ion measurements are planned for the y i n t e n s e nu-

cl ides La, La and Pr. Since these nuclides decay to even-

even daughter nuc le i , we w i l l be able to determine the spins o f levels

in these nuclei (one sp in , the 0 ground state is always known in these

nuc le i ) . These measurements are especial ly important for the levels in1M 1 kb

Ce and Ce, since these nuclei are not accessible for nuclearreaction spectroscopy.

The new computer-based coincidence system ins ta l led in Mainz also

allows three parameter coincidence measurements, i . e . time-Y-Y measure-

ments to characterize metastable states with l i f e - t imes longer than a few

nanoseconds. Such measurements w i l l be real ised for some of the La, Ce

and Pr nucl ides.

An apparatus for measurement of ß-radiat ion and extremely sof t y -

radiat ion (E < 20 keV) is now under construction [Bjö 77]. In th is

apparatus, the aqueous phase leaving the back-extraction step passes2

a th in (<5 mg/cm ) ion exchange f i l t e r onto which the act ive speciesis adsorbed. The f i l t e r is then dried and transferred to the detector

posi t ion w i th in I - 2 s. I t w i l l be possible to choose the sampling

time and the measuring time a r b i t r a r i l y by a sequential ly programmable

timer.

As soon as th is apparatus is in operat ion, we w i l l perform Q.-deter-p

minations on the nuclides discussed in this thes is .

F ina l l y , to obtain a complete picture of the level systematics of

the Z = 58 - 60 nuc le i , an extension of the measurements to the other

side o f the s t a b i l i t y val ley is desirable. These nuc le i , which are

assumed to belong to a deformed region s imi lar to that beginning at N

= 90, may be produced e.g . in heavy ion reactions.

128,

r10. ACKNOWLEDGEMENTS

I want to express my gratitude to all those who have assisted me

in the work preceeding this thesis. First I thank Professor Jan Rydberg

for his interest in and continuous support of my work. Professor Alexis

C. Pappas made the early stages of this work possible by allowing us

to use the experimental facilities at the Department ofaNuclear Chemistry,

University of Oslo. Professor Günther Herrmann kindly allowed us to use

the excellent experimental facilities available at the Institut für

Kernchemie, Johannes Gutenberg Universität, Mainz»

I also want to thank the former leader of the SISAK group, tekn. dr.

Per Olof Aronsson for inviting me to join the early SISAK group in 1972

and also for introducing me to the fields of experimental nuclear

chemistry and use of computers. I also thank the third member of that early

SISAK group, cand.real. Magne Skarestad, for good cooperation during those

years he was a member of the group.

For good and stimulating scientific and social cooperation I express

my sincere gratitude to the present members of the SISAK Collaboration,

viz. civ. ing. .Karin Brodén, Göteborg, cand.real. Tor Bj«5rnstad and

univ.lekt. Eivind Kvâle, Oslo and dr.nat. Norbert Kaffrell, dipl.chem.

Elmar Stender and dr.nat. Norbert Trautmann, Mainz.

I also gratefully acknowledge the assistance of ing. Eva Jomar, Göteborg,

who drew the figures in this thesis.

My sincere gratitude is expressed to fil.kand. Anna Seime, Göteborg,

who performed a thorough test of the H-10 centrifuge and also took part

in some of the Mainz experiments.

The assistance of Mr. Rainer Heimann in the Mainz experiments is

gratefully avknowledged.

I also thank the staff of the Mainz TRIGA reactor for numerous

irradiations.

The excellent technical performance of the SISAK 1 and SISAK 2

systems, can be attributed to Messrs. Hasse Persson, Helge Bratt,

Lennart Bâtsvik and Lars-Erik Ohlsson, Göteborg, who were responsible

for the construction of the different parts of the system.

Professor Gösta Rudstam and fil.dr. Birger Fogelberg kindly

129.

rallowed me to use the OSIRIS mass separator to check the data on La.

I am also deeply indebted to Miss Lehna Andersson and Mrs. Marie

Carlson who typed my manuscript rapidly and carefully.

Richard Warren, Ph»Do, kindly revised the English.

Finally, I would like to thank my father and my late mother for their

continuous encouragement.

This work has been financially supported by the Swedish Atomic Research

Council (AFR), Chalmers Tekniska Högskolas Fond for Ograduerade Forskares

Vetenskapliga Verksamhet, the Bundesministerium für Forschung und Tech-

nologie and the Norwegian Research Council for Science and the Humanities.

130.

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ABBREVIATIONS

AKUFVE

GJRT

HOEHP

I SOL

OSIRIS

SISAK

TDD

VMI

Anordning för Kontinuerlig Undersükning av FSrdelnings-

faktorer vid VätskeExtraktion (Apparatus for continuous

measurements of distribution factors in solvent extraction),

Gas Jet Recoil Transportation

Bis-2-ethylhexylorthophosphoric acid

Isotope Separation On-Line

On-line Separation of Isotopes at a Reactor In Studsvik

Short-lived Isotopes Studied by the AKufve technique

Two detector delay

Variable Moment of Inertia

138.

974, Vol ih. pp I6K<J thWi Pcrgjmon Prcv. Printed in Orcdl HnUtn

SHORT-LIVED ISOTOPES OF LANTHANUM, CERIUM ANDPRASEODYMIUM STUDIED BY SISAK-TECHNIQUE

P O ARONSSON and G. SKARNEMARK

Department of Nuclear Chemistry. Chalmers University of Technology. Fack. S-402 20 Göteborg 5.

Sweden

and

M SKARESTAD

Department of Nuclear Chemistry. Univers > of Oslo. Oslo 3. Norway

iRnmetlWMr 197.1;

Abstract- Shorl-lhcd. neutron-rich isotopes of (he rare earth elements La. Ce and Pr have been studiedb> the means of a continuous solienl extraction separation method (SISAK). Half-lives, energies andrelative intensities are reported for , -rays attributed to 40 ± 3 sec l44La. 20 ± 5 sec ' "La . 11 + 1 sec '4"La.58 ± 3 sec i a7Ce. 45 ± 5 sec l4"C'e and 91 ± 04 sec 150Pr Indirect evidence lor Ihe isolation of Î 5 sec""Ce and 5 10 sec ' 5"Ce is also presented along with upper half-life limits of 5 sec for l4"La and ' '"La.

INTRODUCriON

THE SHORT-LIVID, neutron-rich nuclidcs of hghl rareearths (La, Ce and Pr) are found on the edge ol thedeformed region near the magic numbers /. = 50 andN = 82 Information aboul the nuclear properties ofthese nuelides is of considerable theoretical interest[1-3], and may be obtained only from a detailedknowledge of the decay systematics in this region.

The presently known short-lived, neutron-rich iso-topes of La, Ce and Pr have been produced almostexclusively by thermal fission of uranium[4-9]. Excep-tions are l 5 0Pr and I 49Pr. which were identified fromthe (n. p) reaction in enriched l5oNd[10] and from(;•./)) reaction in natural Nd[4.5]. respectively. Themethods used for chemical separations and decaymeasurements have primarily been of the off-line type,with the exception of OSIRISfll]. However, even thepresent OSIRIS ion source does not allow directseparation of the lanthanides. because of their lowvolatility at those temperatures concerned ( - I500°Cl[12]. In this paper, we shall report on the largelyunknown decay properties of nuchdes in this region,produced tn 14 MeV neutron induced fission of naturaluranium. Rare earth elements are chemically separatedfrom the fission product mixture using the on-linesystem SISAK, described in detail elsewhere[13]. Thesystem utilizes rapid, continuous and automatic solventextraction processes made possible by the AKUFVE-technique[14].

EXPERIMENTAL

Target ami irradiation londtltonsAll irradiations were performed wiih a 14 MeV neutron

generator, manufactured by Philips. The generator givesa stable output of 310'" n sec"', corresponding to aflux of about 310* n cm " 2 sec ' at Ihe tube face.

The uranium target is prepared by adsorption of theUO2(SO4)2 complex, from a solution of |NH4)2SO4 atpH = 3-4, on a Dow ex 1 x 8 resin (200-400 mesh I Theion exchange resin has previously been converted to thesulfate form by treatment with 6 M H,SO4 and largeamounts of 0-25 M (NH4( :SO4 The uranium solution isprepared by dissolving natural UO3 lobtained from the ABAtomenergi, Sweden) in 4 M H2SO4. in the presence ofsmall amounts of HNO3 The loading of the ion exchangeresin is measured by comparing the 185 keV peak of :1<Uwith a known standard, and typically a concentration ofaboul 0-4 mole uranium per liter of ion exchange resin isachieved. About 30 cm3 of the uranium-loaded resin,corresponding to about 3 g of natural uranium, is thentransferred to a target cell (shown in Fig. 1 ). The cell is madeof titanium, and designed according to computer calcula-tions using formulas for the 14 MeV neutron-flux distribu-tion derived by J. Janczyszyn el a/.[15]. These calculationswere performed in order to achieve the maximum integratedflux in the target cell. This type of ion exchange target hastwo important features :

( I ) It facilitates a continuous elution of rare earth productnuchdes using the clutriant 0015 M (NH4)2SO4 However,in spite of the very high distribution coefficient (about IO5|for uranium in the system 0015 M lNK4l,SO4,Do\vex1 x 8[16], a loss or uranium is noticed after some 10 min

1689

P O \KONSSON. Ci. SKARNI WAKK and M S K A R I S I A D

I IJ: I A schematic representation ol tin: target celt usai inthe experiments |1 = titanium w a l k 2 ~ ion exchangeresin. 3 = porous quart / wool plug. -I - sie\e plate. 5 - o-

ring seals. 6 - neutron generator lube Lice)

running In Lict the hall-lilc ol the target, due lo uuuiiuniloss, is of rhe order 20 mm. which necessitates .i recharge olthe target cell afler each run la run UMI.IIK lasts 20 35 miniThis leakage ol uranium is to be expected since the cell ispasset! b> approximate!} K000 IO,(H)(l i_olumn volumes, andprecludes the use of enriched -*"{ . which m a (herniaIneutron flux of about 10 n cm "sec ' lobiamahle with

the neulion generator) would improve the piemen! 1I>MOIIi.itc hya lactor ol 2 3

\2) ft yields a separation ol ihe lesion pitulucis I he noblenase-.. alkali metals, alkaline earth-- and lare e.inh-* .nceluied by 0015 M iNH4)_,SO4. while Zr. Sb. Sh. Sn. \Uf c. Pe. Br and 1 arc retained on the resin I his simplifie-* thesubsequent separation procedure and impnnes the o\ei-.illdecontamination factor

The target running condition- are primarily determinedh\ three factors- (1) pH of elutnant I - 5). l2l temperature olclutnant | - 8 5 ° C | and (3) flow rate ol elutriani (1214 cm* sec ') The \alucs guen m bracket- .ire chosen togju1 Ihe minimum delav lor rare earth michdcs in the targeis\stem h or these nuchdes. the dela\ is ol :he order S -.ce I 3Presumably a higher flow rate would deci MM: the dela\even more, but the present capacities (i| the jnmips andten in lugt-) do not allow a sjgnjiicant increase

C hi-

Cerium The experimental set up for the isolation of radio-acli\e Ce-isotopes is shown schematically in 1 ig 2 1 he ( e-isotopes eluied from the target cell are rapid!} oxitli/ed to( etIV) in a solution of I M H N O , . Ü 1 M I1 ;S() 4 dUi.\0 0 5 M K 2 Cr 2 0- . [ l7 ] obtained b> on-line mixing ol thesechemicals into the eluale A solution ol this compositionwill subsequently be referred to as an m - rim; \oluttnn I heCe is ihen extracted into an organic phase consisuiig ol0 3 M di(2-ethylhexvl|-orthophosphonc acid i H D I H P ) inketosene Subsequently. C ell Vi is reduced and -n tppal inn»aqueous phase using 0 0 2 M sulfamie acid and 0-0S M H ,(>in 1 M H N O j After phase separation in C"2. Celll l] tsreoxidi/ed lt> C'e(IV) m an oxidi/mg solution which thenpasses through an H D E H P PVC column placed in hont ola tie(Li|-delector An accumulation period ol Cc on tlucolumn is followed by dcca> measurement During the-vmeasurements. Ihe decay-products of Ce-nuchdes ure con-tinuously eluied by an oxidizing solution, in order tominimize any interference in the -,-spectra

Column muterutj is prepared b> <Jurr}i»g 21)g ol PVCbeads 150 KK) meshj m ti mixture consisting ol i\ ml pure

0 * M HDEHP

0 0IM (^JH4)2SO4

PH 4-5

acid

I M HNOj

0 IMH?SO4

0 05M K?Cr?07

I M HM03

0 I M H J S04

0 0 5 M K-,Cr2O7

i M H? S04

0 05 M H202

I M lactic aod

Hg 2. Experimental set up for the separation and identification of Ce-isotopcs (n -- neutron beam, [P =fission products. Cl. C2. O = mixer-centrifugal separator units. [> ~ detector)

rShort-lived isotope1* of lanlhanum. cerium and praseodymium studied by SÏSAK-tcchmque

0 3 M HDEHP in Kerosene

I (SSI I

0 01 M (1\JH4)2 S0 4

pH 4 - 5 2 4

0O5MH?O2

IM lactic acid

Fig. 3 Experimental set up tor the separation and identification of La-isolopes, according to the delaymethod (n = neutron beam. I-P = fission products. CI. C2. C3 = mixer-ceninfugal separator units.

D = detector. DFC = detector flow cell).

HDEHP. 12 ml toluene and 20 ml chloroform. Alter theorganic diluents have evaporated, ihe column material isready for use.

Lanthanum. The experimental set up for the isolation olradjoacuve La-isotopes is shown in hig 3 The pH of ihesolution emerging from the target cell is adjusted lo! 40 ± 005 b> continuous addition of 0-25 M HNOA An>trivalent lanthanides present (maini> Lu. Ce and Pr» .irethen extracted into an organic phase consisting of 0 3 MHDEHP in kerosene. The phases are separated in C I. andLa (plus Prf is back-extruded to an aqueous phase b\adding an oxidizing solution, which leaves Ce||V| m theorganic phase. The /-measurements are then carried out onthe aqueous phase leaving C2. preferably according to thedelay methodf 13] since the La isotopes ofintertfsi arc-expec-ted to have half-lives below one minute If higher decon-tamination from Ce is desirable (e g. for grow mg-tn measure-

ments), the solution is passed through an HDEHP PVCcolumn prior to the /-measurements

Praseodymium. The experimental set up for the separationof Pr is in principle similar to that for the separation of Ceand La. and is shown \chematically in Fig 4 Cc(IU) isoxidired to CellV) and extracted into an organic phaseconsisting of 0-3 M HDEHP in kerosene (in Cll Thengroun-in Pr-nuclidcs m the organic phase are transferredto an oxidizing solution in C2 This solution passes througha Dowex 50 W x 8 (50 10« meshl column, where Pr and itsdaughters are adsorbed. The accumulation period is followedb> decay measurements. The sane set-up also facilitatesmeasurements according to the delay method In this case,the Dowex 50 column is left out The separation system forPr was designed with a Mew to obtaining indirect identifica-tion of shorl-hved isotopes of Ce.

Genera) comments on ihe i hemxt a\ prm edure\ The organic

0 3 W HDEHP ,n kcosene

0 01 MINH^SO«pH 4-5

I M HN03

O I M H?S0«O 05 M Kfrz07

0 I M H2S0„0 05 M K Ci O

I M H? SO,

0 05M Hg0g1 M lactic acid

Fig. 4. Experimental set up for the separation and identification of (he daughter products from the decay ofCe-isotopes in = neutron beam. FP = fission products. Cl. C2. C3 = mixer-centrifugal separator units,

D = deteclor).

riw: O ARONSSON. Ci SKARMMAKK and M. SKAKESIAD

plusc of l) 1 M IIDL:HP m kerosene runs in a closed circuitin .ill cx]Krimcn!s This, because il is essential to minimizeill«.- ciinsumplion ol chemicals and Ihe amount ol wasteI he cunlinuous recycling of the organic phase necessities.1 conditioning slep in order lo prevent any accumulation ofkmg-li\ed species. This is done by contacting the organicphase « ith an aqueous phase consisting of 1 M H2SO4.1 MIdctic acid and Ü-05 M H2O2. as illustrated in the flowdiagrams (Kigs. 2 4).

"1 he over-all chemical yields are generally higher than50 per cent for Ce and higher than 85 per cent for La and PrThese have been measured in off-line tracer-experimentsThe transport timefl.1] from target to detector is found tohe of the order 10-20 sec. depending on the number ofseparation stages involved and on the flow rates used.

Che/ntcut-* Technical grade chemicals were generallysufficient (eg. in the case of acids. K2Cr2O7. H2O2).(NH4I;SO^ was p.a. in order to avoid contamination fromactivated impurities Dowex ion exchangers of technicalgrade were purified before use PVC beads were supplied byKcma Nord AB. Sundsvall The HDEHP »as supplied byFarbenfabriken Bayer AG. Leverkusen, and used withoutpurification Shell-sol TD was used as organic diluent andion-exchanged water was used 'hroughout.

Counting equipment and data evaluation. The detection ofy-rays was carried out using a Ge|Li)-detector systemdescribed in [13]. The /-spectra were analyzed using thecomputer programs SAMPO[18] and GAMANL[I9]. Half-lives of /-peaks were determined both graphically andnumerically according to the least-squares method.

RKSULTS AND DISCUSSION

Short-hri'tl isottijtcs of I.;t

A ,-ray spectrum of the La-fraction is reproduced inKtg. 5. The prominent y-peaks are listed in Table 1according to the half-lives, and attributed to the iso-topes 144La. 141>La and 14BLa. A half-life of 41 sec for'4 4La has recently been reported by Ohyoshi a u/.[7].based on the decay of the /-rays at 397 keV and541 keV. The mass-assignment is in accordance withthe results of Cheifetz el u/.[20l However, in theexperiments of Ohyoshi el ai. the time between irradia-tion and counting amounted to about 5 half-lives for'•"La. Presumably this is the reason why the lessabundant y-rays at 165 keV. 585 keV. 845 keV were

Table 1. Half-lives and ;-ray energies of La-isotopes.Relative intensities of y-rays II.) are given in bracketsUncertainties in /,. is 0-2 keV and in £ 10-20',',, respectively

/ keV(£ ."„1

165-31141.397-3(1001.5411(42).585 01 ! l).8449(29)1190(100). 170-5 (52). 189-2 (59)258-5 ( 100), 409-7150). 502-9 (25)

Nuchde

' • " L a

' • " L a'•"•La

7",

40

2011

,(secl

+ 3

1+1+

Pig 5. v-ray spectrum of the La-fraction. Energies are given in keV. The experimental set up is shown inFig. 3

0 10° -

Short-lived isolupcs of l;imh;iiHim. cerium anil praseodymium studied by SISAK-tcchniiiuc

nul delected In addilum. M have observed weak,-lines :it W>, AM). 44h. 4X4 and 494 keV. which(cr.tutivuiy .lie assigned In l 44La

So far. no conclusive data have been presented lorthe isotopes 145La and l4 ' 'La. However, informationavailable from other laboratories along with our owndata suggests that the /-groups with half-lives 20 + 5 secand 11 ± I sec should be associated with the decay ofl 4 'Laand ''"'La. respectively.

A brief summary of the data available on l 45La isgiven below :

Grapengiesser et al [9] have reported a componentin the 145-mass-cham. decaying with a half-life of36 sec The element suggested is La. in accordancewith the findings of Wilhelmy[2l].Seyb[22] has. in a preliminary experiment, indirectlymeasured the half-life of l 45La to be 28 + 3 sec.Our delay-measurements show that the decay of14SLa is accompanied by the growth of l45Ce (y-peakat 724 keV). The grow th curve indicates a half-life ofthe order 30 sec.On this basis, we have assigned the y-rays at 119. 17Ü

and 189 keV to the nuchde 145La.The /-energies 258. 410 and 5(13 keV decaying with

half-life 11 + 1 sec show excellent agreement with theenergies of the three ground state rotational levels of14"Ce reported by (.'helfet/ et <//.[20]. This makes theassignment of the 11 sec /-rays to '4 hLa unambiguous.The decay of the photo peaks is shown in Fig. 6.

Cheifet/ et al. also report data for the ground-staterotational levels of l4"Ce. However, the corresponding;-rays have not been detected in our experiments,indicating that T, ,|148I.a) < 5 sec. This limit is sup-ported by the fact that no growth of '4"Ce was observedin any of the delay-measurements. Likewise, theabsence of any growth of '4"Ce indicates that the sameupper limit also applies to the half-life of l4"La

I-

^ ^

• 409

• II 7

keV. T '\Q

%ï I Oiec

? ï 0 6sec

Deio* time, sec

Fig. 6. Dc-eaj curves of the 258-5 keV and 409-7 keV y-raypeaks present in the La-fraction Q is the ratio between the

photo-peak ureas in the two detectors.

Dalay time, sec

Fig. 7. Determination of the half-life of "°Pr. based on adecay plot of the 131 keV photo-peak present in the La-

fraction.

The conclusions drawn for 147La and 14sLa are inagreement with the preliminary measurements ofSeyb[22]. which indicate half-lives of the order of I secfor both nuclides.

An examination of the y-spectrum in Fig. 5 showssome contamination by nuclides of the noble gases,particularly l39Xe and 90Kr. We were able to makeallowance for this contamination, from subsidiaryexperiments in which the HDEHP was left out of theorganic phase.

The y-ray at 131 keV indicates the presence of 15"Pr.The decay of this photo-peak is shown in Fig. 7 and theresult of 91 ± 0 4 sec is slightly higher than thereported value of 61 ± 0-3 sec[IO].

Short-lived nuclides of Nd and Pm are expected tobe present in the La-fraction. However, the fissionyields of these nuclides are relatively low and we havenot been able to make any identifications of knownnuclides in this region.

Slum-lweil isotopes o / Ce.

The y-ray spectrum ( Fig. 8) of a Ce-fraclion shows ahigh radiochemical purity obtained by employing theHDEHP/PVC column (see Fig. 2). The contributionfrom growmg-in Pr was kept to a minimum all the limeby a continuous elution with an oxidizing solution.This could be done because Ce was not cluted, as wastested by measuring the half-life of photo-peaksbelonging to the isotopes "*5Ce and 146Ce. In thisconnection, it was noticed that a small fraction of thePr formed on the column seemed to be retainedpermanently, without being affected by the elutriantat all. Presumably, this is due to channelling.

P. O. ARONSSON. G. SKARNEMARK and M. SKARESTAD

Kig. 8. ,-Ray spectrum of Ce-isotopes recorded 0-30 sec after the end of a 3 min accumulation. Energies aregiven in keV. The spectrum was obtained by summing measurements from 14 separate runs. The experi-

mental set up is shown in Fig. 2.

The column experiment illustrated in Fig. 2 is wellsuited for the identification of the -/-rays associatedwith the decay of " ' C e and 148Ce. These isotopeshave reported half-lives of 65 sec[4] and 43 sec[4],respectively, but so far no assignment of -/-rays hasbeen presented. According to our decay-measurementsa number of photo-peaks decay with half-lives between40 and 70 sec. The two intense -/-lines at 269 and293 keV with half-lives 58 ± 3 and 45 ± 5 sec areassigned to l47Ce and [48Ce, respectively, in agreementwith the results of Seyb[22]. However, the smalldifference between the half-lives of 147Ce and ;4*Cemakes further assignments difficult. Consequently, inTable 2 we have listed all photo-peaks with T,l2 in theregion 40-70 sec in the same group. The intensitiesquoted are normalized to the prominent photo-peak at269 keV.

Our column experiments (Figs. 2 and 8) show noV-rays which conclusively may be attributed to l 4 9Ceor 150Ce though y-lines at 93, 141, 168. 478 and555 keV decay with half-lives 25 ± 10 sec. However,we have some indirect evidence for the presence of'•"Ce through its decay-products 149Pr and 149Nd.These nuclides were identified on the Dowex 50column in the experimental set up in Fig. 4. Thepresence of 149Nd is illustrated in Fig. 9.

Table 2. /-Rays observed in the Ce-fraction on the HDEHP/PVC column. The corresponding "/-spectrum and experi-mental set up are shown in Figs. 8 and 2. respectively.

Uncertainty in £ is 0-2 keV

£,.keV

99-2105-4117-3121-4196-0218-6::69-5273-8292-6325-236003696374-7390-4422-3452-5467-6580-86061700-7

(sec)

51 + 355 + 545 + 253 ± 350 + 456 + 558 ± 350 ± 645 + 559 + 355 + 550 + 559 ± 342 ± 530 + 1050 + 555 + 560 + 360 + 438 + 6

o51 ± 79 + 29 + 2

37 ± 75 0 + 730 + 6

100+ 1022 ± 599 + 1025 ± 58 + 25 + 2

16 + 47 + 2

15 + 427 + 618 + 420 ± 59 ± 3

10 + 2

rShorl-livcd isotopes of lanthanum, oenum and praseodymium sludied by SISAK-icchmquc 1695

Fig. 9. A y-ray spectrum, illustrating the presence of 140Nd in the decay product fraction from decay ofCe-isotopes. The y-spectrum was recorded 70-90 min after the end of the accumulation Energies are

given in keV.

The decontamination factor with respect to Pr in (hefirst extraction stage has been evaluated in separateexperiments. The factor was found to be ~ I03. whichexcludes the possibility that ' " N d originates fromprimary extracted " ' P r or 14*Nd. Consequently, thepresence of '* 'Pr and '*'Nd must be due to the decayof ' '"Ce in the delay between Cl and C2. This impliesthat the half-life of 149Ce ä 5 sec.

The presence of the y-peak at 131 keV from ! S 0Pr inthe Ce-fraction in the organic phase indicates that150Ce has been extracted at Cl . On the basis of thetranspon time involved, we presume thai ' 50Ce has ahalf-life of some 5-10 sec.

Acknowledgement—The authors are indebted to Drs. E. Ednand R. P. Schumann for their contributions during the earlystages of the investigations. Dr E Kvâle assisied skilfully inthe nuclear measurements and data handling We are alsoindebted to Professor J. Rydberg. Professor A. C. Pappasand Dr. J. Alstad for valuable suggestions. Miss C Hard afSegerstad typed our manuscript admirably The SwedishAtomic Research Council and the Norwegian ResearchCouncil for Science and Humanities gave financial support.

REFERENCES

1. D A. Arseniev. A. Sobiczewski and V. G. Soloviev,Nucl. Phys. A139. 269(1969)

2. S. G. Nilsson. C. F. Tsang. A. Sobiczewski. Z. Szy-manski, S. Wycech. C Gustafsson. 1. L. Lamm. P.Möller and B Nilsson. Nucl. Phys AI3I. 1 (19691.

3. G. Gneuss and W. Greincr. Nucl. Phys. A17I. 449(1971).

4 D. C Hoffman and W. R Daniels. J. morg. nuclChem. 26. 1769(1964).

5. J. van Klmken and L M. Taff. Nucl Phys 99. 473(1967).

6 J L. Fasching. Thesis. MIT (1970).7. A. Ohyoshi. E. Ohyoshi. T. Tamai and M. Shinagaua.

J. morg. nucl Chem. 34. 3293 (1972).8. D. C. Hoffman. H. F. O. Lawrence and W R. Daniels.

Phys. Rer. 172. 1239(1968).9. B. Grapengiesser. E Lund and G. RudMjro. In Ptoc

Int Con/, on lhe Properties of Nuclei Jar from the Regionof Beta-Slabihlr. Levsm 1970. CERN-Report 7(V 30.p. 1093(1970).

10. T. E. Ward, N A Morcos and P K. Kuroda. Phys.Ret Cl, 6. 2410(1970)

11. S Borg. B Rydberg. L E de Gcer. G Rudstam.B Grapengicsser. E Lund and L Westgaard. A Wlnstrum. Meth.91. 109(1971)

12. W. E Talberl. in Proc. lut Con) on ihc Properties ofNuclei far from the region oj Beta-Stability. LevsinWtt.

CERN-Report 7a 30. p 109 (1970).13. P. O Aronsson. B. Johansson, i. Rydberg. G. Sliarne-

mark. J. Alstad. B Bergerscn. b. Kvilc and MSkarestad. J. inorg. nucl Chem. In press.

14. C. Andersson. S. O Andersson.J O. Liljenzin, H Rein-hardt and J. Rydberg. Acia chem. uanJ. 23. 278!(1969)

15. J. Janczyszyn and L. Loska. Radiochem RadioanaiLett. 3. 343 (1970).

KWh P. O. AHONSSON. Ci. SKAKNUHAIIK und M SKAKKSIA»

16. K A. Kraus and F. Nelson. Sympnsmm an tan Ex-change and Chromatography in Analytical Chemistry.Special Technical Publication No 195. AmericanSociety for Tesling Materials (1958).

17. G. W. Smith and F. L. Moore. Analyi. Chem. 29. 448(1957).

IX. J.T. Routti and S. Ci. Pruv-in. Nucl. Imi M.r/i 72. i:<(1969).

19 R. Gunmnlt. UC1D-I514O<1967).20. E Cheifelz. J. B Wilhelmy. R. C. Jarcd and S. G

Tnompwn. Hm. Rci.4C. 1913 (1971).21. J. Wilhelmy. ÜCRL-18978 (1969).22. K E. Seyb. Private communication (19731.

rSISAK -A NEW TECHNIQUE FOR RAPID, CONTINUOUS

(RADIO)CHEMICAL SEPARATIONS

P. O ARONSSON. B. E.JOHANSSON. J RYDBERG and G. SK ARNEMARKDepartment of Nuclear Chemistry. Chalmers Unisersitv of Technology. Fack. S-402 20 Göteborg 5.

Sweden

and

J. ALSTAD. B. BERGERSEN. E. KVALE and M. SKARESTADDepartment of Nuclear Chemistry. University of Oslo. OsJo 3. Norway

IRecmvd 19 October 1973)Abstract SISAK. a new continuous technique for on-line chemical separation of short-lived species, ispresented. The system features multistage luo-phasc liquid liquid and liquid solid separations completedwithin 3-5 sec per stage. The system is easil\ adaptable for selective isolation of almost all elements Theoperational characteristics of the system are outlined and the delay properties briefly discussed. A two-detector delay method for half-life determinations is also presented. The application of the technique to thestudy of short-lived nuchdes is exemplified by lesults from investigations of 30 sec ^"'Cu. 3-8 min6Sl"Cuand42sec™Cu.

INTRODUCTION

THE STUDY of the properties of nuclides far off the/(-stable region has many important theoretical aspects[I] and several new and improved experimental tech-niques have been developed in order to provide thenecessary experimental data [2]. The most comprehen-sive systems leature an isotope separator on-hne(1SOL) facility, which offers the advantage of fast andalmost unambiguous mass separation of productnuchdes. An elemental separation, however, is onlyachieved when the vaporization of products in thetarget is selective.

A development in this area is the improvement offast radiochemical procedures by automation, mainlyby Herrmann and his group[3]. In these wet chemistrymethods, the ; lemental separation is usually reliable.A disadvantage, however, is the fact that their pro-cedures are carried out discontinuously[4].

Solvent extraction is a technique of high potentialvalue in the development of fast chemical separations.This is because of the rapidity of the actual extractionprocess and the high chemical selectivity offered. Theslowest step in this procedure is that of phase separa-tion. However, this difficulty has recently been over-come by the development of the so-called H-centrifuge[5]. In the present system, named SISAK.* continuousmultistage chemical separations of liquid phases areaccomplished by H-centriftiges. The hold-up tune perstage is short, and offers the opportunity of separatingnuclides with half-lives down to a few seconds.

* SISAK, i.e. Short-lived Isotopes Studied by the AKufvc[6] technique.

In this paper, we shall describe the main propertiesof the SISAK-syslem, including a new dynamic flowmethod for half-life déterminations. The application ofihe sysîem to (n, p)-reactions in a Zn-target will also bedescribed. Other results obtained using the SISAKtechnique are given in refs. [7, 8].

SISAK EQUIPMENTAND EXPERIMENTAL TECHNIQUE

The SISAK arrangement consists in principle ofthree main parts (Fig. 1): the target system (irradiationsource and target cell), a chemical separation system(mixers and centrifugal separators), and a nucleardetection system (collectors and detectors). The detec-tor system is connected to data recording devices.

Target system

Irradiation source. Our present irradiation source isa l4MeV Philips neutron generator, PW532O. Itfeatures a vertically mounted sealed-ofT acceleratortube with a self-replenishing tritium target. At 150kVit gives a stable output of 3 x 10'°nsec" ' , corres-ponding to a I4MeV neutron flux of some 3 x 10'ncm " 2 sec " ' at the tube face.

Targets. The target should have properties whichfacilitate:11) a continuous production of the product nuclide

over an acceptably long period of time,(2) a continuous, rapid and efficient separation of the

product nuclide from the target without (severe) lossof target material,

(3) a large amount of the target nuclide exposed to themost intense part of the particle beam,

2397

P. O. ARONSMIN et al.

14) a minimum release of interfering activities from thetarget. (In our ehcmiea! system this is not a require-ment, although advantageous.)

In SISAK two different types of targets have so farbeen developed, the homogeneous liquid target and theion exchange target. The liquid target simply consistsof a cylindrical titanium cell through which the targetsolution is pumped. The cell is mounted at the end ofthe accelerator beam tube. Examples of this type oftarget are given in ref. [7] and in the Cu-expj'imentdescribed below.

The second type of target may be exemplified by theUO2(SO4)2 ~ complex adsorbed on an anion exchanger.This target was used in the study of short-lived La, Ceand Pr-isolopes[8].

Chemical reparation system

For an effective mixing of the two liquid phaseswithin a short contact time (< 0-5 sec), a static mixer(Kenics Corp., U.S.A.) is placed ahead of each centri-fuge inlet. The centrifuges, mounted in ventilated racksand made of titanium, are driven by asynchronouselectric motors. The maximum flow rate through acentrifuge is 15-20cm3sec"' per phase.

The purity (i.e. freedom from entrainments) of theoutgoing phases is an important factor affecting theradiochemical purity of the final sample. If the centri-fuge is running at extreme conditions (e.g. maximumspeed or flow rates), some entrainment may occur,especially during start-up. The entrainment, which isusually traces of heavy phase in the light one, iseliminated or minimized by increasing the pressure onthe "impure" phase. The pressure is adjusted byremotely controlled throttle valves.

The flow rate of solutions fed into the system isadjusted by pumps of cog-wheel type, lubricated bythe liquid itself. The pump-houses are made of titaniumand the cog-wheels of Kel-F (manufactured by Fluoro-carbon Co., U.S.A.). The pumps are equipped with d.c.motors and a continuously variable voltage supply inorder to achieve good regulation properties. One pumpis specially designed as a booster pump and is placedahead of extraction and ion exchange columns.

All tubing in the SISAK system is of polytetrafluoro-ethene (PTFE : usually of i.d. 4 mm) to achieve chemicalinertness and low adsorption of radioactive species.

The composition of a liquid phase is checked byglass or redox electrodes, placed in specially designedflow cells[9]. Corrections of the composition may beperformed continuously by automatic titrators.

The temperature of the solutions is adjusted by theuse of heal exchangers connected to a thermostaticbath. The temperature is determined by thermistors.Elevated temperatures are used to increase reactionand diffusion rates, and to decrease the viscosity, whichwill lower the minimum flow rate required to obtainplug flow[10], In addition, removal of the noble gases

by blowing nitrogen into the mixer is improved utelevated temperature.

The equipment is remotely controlled from acentral control panel which also features instrumentsfor flow and pressure readings. Remote control allowsa single person to operate the system (cf. Fig. 4) andminimizes radiation hazards.

Product colleclion and measurement of .sampler.

Product colleclion. The product nuclides of interestare concentrated by the following methods :(1) Isotopic exchange on a preformed precipitate[ll].

This technique was used in the Pd Ag studies[7].(2) Electro-chemical reduction. In the Cu-experiments

described below, Cu was reduced on iron wool[12].(3) Adsorption on an ion exchange column. In the

lanthanide experiments[8], a cation exchanger wasused to retain trivalent lanthanides.

(4) Colleclion on an extraction column. This methodwas used in the lanthanide experiments for therecovery of tetravalenl Ce[8].

Due to the selectivity of the retention techniques,these methods also give improved radiochemical purityof the samples. Alternatively, measurements may becarried out directly on a liquid phase. This is facilitatedby the use of a spiral tube cell, surrounding the detectorhead.

Measuring equipment

At present, the SISAK nuclear detection system featurestwo ORTEC Ged-O-detectors. both with resolution 2 0 keVat 1332 keV and with relative efficiencies 3-6 and 7-8 per centrespectively. The detectors are shielded by 10 cm lead andconnected 'via ORTEC amplifiers (model 4S4) and LabenADC s (4 k and 8 k, model number 8210 and 8211, respec-tively) to a 16 k Nord-1 computer, serving both as multi-channel analyzer and data handler. The data stored may bedumped on a magnetic tape within 5 sec The evaluation ofy-spectra (peak location, resolution of multiplets, encrgvdetermination and compulation of areas) has been per-formed by use of the computer programs GAMANL[I31andSAMPO[14].

With the view of evaluating ß-vatues by ß- y coincidencemeasurements, this may be feasible by introducing equip-ment for detection of /J-particles in a liquid phase (e.g.Cerenkov detectors, liquid scintillalors, glass and plasticscintillators).

Delay properties

In an experimental system designed for the study ofshort-lived nuclides, evaluation of the many factorsthat influence the delays in the system is of importance.In SISAK, the delays are caused by liquid hold-upvolumes of various kinds. An exact solution of thehydrodynamical equations governing the liquid flowis not possible, and idealized hydrodynamical modelsfor tubes, target cells, centrifuges and detector cells aretherefore used. These models are based on the assump-tion of two extreme flow patterns, plug flow andthoroughly mixed flow.

The main purpose of the models is to predict

rSISAK a lechnique for (radiojchcmical separat

! Product collection and| nuclear detection system

Fig. 1 Principal design of the SISAK system. LFCS = liquid flow centrifugal separator. D = dctccior.DC = detector cell (with or without collector). The chemical system may be extended by several mixer-

separator stages, and the liquid phases may run either in open or closed circuits.

Iransmission factors for lubes, centrifuges, etc. and tooptimize the count-rate of a desired nuchde withrespect to competing activities. The transmission factorIt) is defined as the fraction of a radioactive specieswhich survives the delay caused by liquid hold-upvolume(s|[IO].

Fig. 2. Ihe inset shows a typical configuration of theSISAK system. TC = liquid target cell, thoroughly mixedvolume l30cm\ DTI = transport tube Iplug flow volume12cm3) between target cell and C = centrifuge Iplug Howvolume 12 cm3, thoroughly mixed volume 35 cm3). DT2 =transport tube of plug flow volume 35 cm3 (for transportthrough radiation shield to detector or subsequent centrifugestages. / = liquid flow rate, 15 cm3 sec"1, n = neutronbeam. The curves show the totai transmission factor for thisarrangement and for arrangements with totally 2, 3 and 4

centrifuges.

The inset in Fig. 2 shows a typical arrangement witha homogeneous liquid target cell, two delay-tubes and acentrifuge (also cf. Fig. 4). The curves in Fig. 2 show thetotal Iransmission factor as a function of nuclide half-life for this arrangement and also for arrangementswith 1 3 and 4 centrifuges in a multistage system. It isevident that the transmission decreases with anincreasing number of centrifuges and a decreasingnuclide half-life. Thus for a 3 -sec half-life, T IS about

1 1

T, ,>8 2510 15sec

IX 1

1

Af—A PI 1—\

T

1

• 108 6 keV8 6 sec 9lKr

39 7 sec " xe

-

i^J" .'3S8îl0s€c

A -P |-—| P2 |— tT T?

20 40 60 80

Delay time, sec

Fig. 3. Decay curves for the 108-6 keV peak of ^'Kr and296-6 keV peak of 139Xe obtained with the two detectordelay method. Q is the ratio of peak areas in detector 2 andI, respectively. The inset schematically shows the two detec-tor delay arrangement. Dl, D2 = detectors, PI, P2 = detec-tor flow cell volumes, cm3; P = delay volume, cm3; / =liquid flow rate, cm3 sec ' and XTlfT2 = hold-up time,

sec;(T= P//).

I) AKOVISIJ\ fl/jj

14

n(2)

14

n(2)

15

(I

8 128 12E

eE

7

12

12

12

Pig 4 General plan of the S1SAK expérimenta! area at Ilie Department of Nuclear Chemistry. University ofOslo 1 = paraffin. 2 = boric acid paraffin. 3 - concrete. 4 = walcr filled observi "ion window, s = neu-tron generator beam tube. 6 = HV and power supplies for the neutron generator. 7 = neutron generatorcontrol panel. 8 = pump. 9 = mixer-centrifugal separator unit. 10 = power supply for the ce Unfuges.Il = central control panel. 12 = solution storage vessel. IJ = Ge(Li)-detecio' = lead shield. 15 =nuclear electronics. 16 = pH-meter. 17 = temperature controls. 18 = hca. .. angers. 19 = waste

collection

01 with one centrifuge laboul 10 per cent of thenuchdes sumvel, but only 001 with four centrifuges.On the other hand, for nuclides with half-lives S IOseeseveral centrifuges may be introduced without tooserious losses. The predictions in Fig. 2 are in ac-cordance with experimental findings.

Corrections for chemical yields Igenerally iW peicentl are not included in the transmission factor.

*\ detailed discussion of the delay properties ispresented elsewhere[IO].

]Hi)-ih'[ft Inr di'ltiv nwlhtht fur Ihitj-lili' itewnniiuiliuii

In order to allow convenient half-life determinationsin ihe range 1 100 sec. and to lake .ulvantage of thecontinuity of the SISAK-sjstem. a dynamic flowmethod has been developed. The method is an exten-sion of the one detector delay method described byCampbell and Nelson[l5], hut it has the additionaladvantage of being insensitive to source strengthfluctuations. In the present method, two /-detectorsare used along with two detector cells and a delay tubeof variable length between the cells. With the symbolsused in Fig. 3. and according to the mathematicalmodel for the flow properties of ihe system[lüj. theratio between the count-rates m detector I and _.respectively, is expressed b)

n o B ' •Ae*P[ - ' - lT i + 71]

where 6 = 1 - expl-ÀTl. i. is the detector efficiency,and subscripts 1 and 2 refer to detectors 1 and 2.respectively. This ratio is a function of /» T,. 7 and Tfor a given set of detectors and detector cells. However.

if f, = A. i.e. the two '. x i t r '.Is ve ''en'1 al. theformula simplifies to

Q = ''2 e \ p r - ; . i T , + T)]' • i

which is a function of / and 17", + 7~l only. By per-forming experiments at various T, + 7 IT varieswith the tube length between ihe two detector cells) adecay curve is ohtained.

The method has been tested on kerosene solutionscontaining noble gas nuchdes. The decay-curves lorpeaks belonging to 4 I Kr and ' 'lliXe are shown in Fig. 3ami Ihe half-lives obtained agree fairly well »nil thosein ihe Iiterature[l6].

A point .it r, + 7 = 0 is always given by the ratiool the detector efficiencies n.- i.-i This point can beohtained by calibrating the detectors with a solution ola long-lived nui'ide.

Similar, but more elaborate equations can bederived for growirTg-in activities, though their applica-tion to half-life determination is more complicatedThe two-detector delay method, however, reveals anyoccurrence of growing-m activities.

STIDY OF SHORT-LIVED Cu-ISOTOPES

The SISAK technique was initially tested on theseparation of short-lived Cu-isotopes produced byin. m ri-actions in Zn. This system was chosen in orderto investigate the nuclide ""Cu with an estimated half-life of 5 M) sec. The estimate is based on the <?-valuefrom Sceger's mass table[17]. The experiments werealso expected to provide additional information onother short-lived Cu-isotopes. e.g. "8Cu <T, 2 = 30 sec)[18]. A view of the experimental area is given in Fig. 4.

rSISAK a technique for (radiokhcinical s 24IM

5*4 LIX -64 m

Fig 5 Flow diagram for the separation of short-livedCiMiotopes n = neulron beam, CI, C2, C3 = mixer -

ceninfugal separator units, D = detector.

ExperimentalTarget and irradiation. All irradiations were performed at

the neutron generator described above. The target was a6 M solution of ZnCIn in a cylindrical titanium cell (volume.ibuul 130 cm3, corresponding to some 50gZn). In order todecrease the viscosity of the target solution, the temperaturewas maintained at 35 40°C. The flow rate was usually about12 cm1 sec" '. With this target arrangement, the productionrate of 7"Cu was estimated to some 7 x 10* atoms sec' '.assuming n formation cross section of 7 mbarn[19,20].

Chi'inual procedure. After irradiation, Cu was extracted(Fig 5\ almost quantitatively into a solution of 5 per centLJX-64 in kerosene. /-Measurements on the organic phaseafter this first step revealed the presence of small amounts of" (Zn. ""Zn. «Ni. 34CL 37S and 23Ne formed by various neuIron reactions with the target solution. Although the solubility

of water in kerosene i* low ( -0-01 per cent ill 25"Olj21 '. thepresence of Zn. CL S and N in the organic ph.isc is due t«>this solubilily The Nc is nuniK extracted bcciusc <*1 Usaflinily to alkanes. The NL on the other hand, is p.nti.ill>extracted (£ - 40 per cent» b> LIX-64.

A washing step (C2) of the organic phase with H S O ,(pH = 2) removes Zn. Ni. CL S and most of the N. This u j*hwas followed by back-extraction of Cu into OS M HClFinally Cu was collected in frunl of the detector by reducimnon an iron wool column. Ne remained in the org<tnii phjscand did not interfere.

After some 30 mm running, transport of acid from C2 toC3 and from C3 to Cl was found to influence the separationprocess. The measured transport-rates for the acids betweenthe two phases correspond closely to those expected fromthe flow rates and the solubility of aqueous phase in keroseneThe pH was adjusted by adding H^SOj and N;iOH fromtwo pH-regulated til rat ors (cf. Fig. 5|. This permittedcontinuous running for hours without drift of pH.

For Cu, the equivalent transport timc[K)j from urgcithrough three centrifuges to the detector WJ.S estimated tu20 sec. allowing measurements of nue! id es with half-li^esdown to about 5 sec.

Chemicals. Commercial grade LIX-64 manufactured b>General Mills was used without purification The rcjgcniis a mixture of LIX-63[29], a dialkyl hydroximc. and 2-hydroxy, 5-alkylbenzophenone oxime[30.3l]. Shell Sol-Twas used as organic diluent. The iron wool was of technicalgrade and was rinsed by acetone. Other chemicals were ofp.a. grade. Ion exchanged water was used throughout.

Nuclear measurements, '/-measurements were performedwith the detection system described above. In these experi-ments, performed at an early stage of the SISAK-proiect.the program GASP-8[22] was used for the evaluation of the

H 111111 n n j i T1

1400 1600 1800200 400 600 600 1000 120'

Channel number

Fig. 6. A gamma ray spectrum of the Cu-fraction on the iron wool column (cf. Fig. 5). The Ga and Gepeaks are due to neutron interactions in the detector during irradiation.

r24(12 P. ( ) . AROMSSONff «/.

M.ilf-livt's were determined by measuring con-single specira. and dec.iy c u n c s were analyzed

grapliK-.ilK

Rt'^utr\ Litnt discussion

A typical /-spectrum, recorded during accumulationof Cu on the iron wool column, is shown in Fig. 6.Energies and half-lives are listed in Table 1.

The dala oblained for ""Cu. 6<ICu and 67Cu are ingood agreement with published data[23,24].

Decay curves for some of the photo-peaks previouslyassigned to """Cu showed two components with half-lives 30 sec and 3-8 min. This made us consider thepossibility of an isomeric slate in 6sCu. During theseexperiments. Ward et al. [25] published their data whichassigned (he 3-8 mm activity lo the isomer. A recentpublication by Singh et a/.[26] reports the same findings.

The gamma peaks at 885, 902 and 1252keV decaywith half-life 45 + 15 sec, and as the lowest excitedstate in 7"Zn Is 884keV[23], we assign them to the newisotope 7nCu. These y-rays have previously beenassigned to """Ni by Meason and Kuroda[27], but onthe basis of our chemical separations we are able toexclude any Ni-activity.

During these experiments, Taff et al.[28] publishedtheir results on 70Cu. obtained by (n, pl-reaclion on a78.1 per cent 7nZn-target, but without any chemicalseparation. They found several /-lines decaying withhalf-lives of 5 sec and 42 sec. These -/-lines were

Table I. Gamma lines observed in the decay of Cu-isotopes.Is = strong, w = weak, vw = very weak)

EnergyIkeV)

8S4-yMOI 6

1251 584 5

111(1526-?578-5636-88060

1077-1126I-I1340-3

* 0-2+ 0-2± 0-3

+ 0-2+ 0-4+ 0-8+ 0-3+ 0-3+ 0-3+ 0-4+ 0-3+ 0-3

1745-3 + 0-21883-5

578-S8060

1077-11261-11745-31883 5

93-2184-0

«33-21093-3

51101345-6

±0-2-t-0 3+ 0-3+ 0-4±0-3+ 0-2± 02+ 02i 02+ 0-5± 0-3+ 0-2±0-3

Intensity Half-life

ww 45 ± 15 secw

sswsvw 3-8 + 0-I minsH

sswvws 30 + 3 secs

s

ww 590 hr*

wi 51 + 01 minsw l2-8hr*

Assignment

7 0 Cu

6<""Cu

h BCu

"Cu

*6Cu

"Cu

•Fromrel. [23].

attributed to 70"Cu and 7l"'C'u. respectively. The resultsof TalT et al. along with our experiments, make theidentification of 42 sec7nl'Cu unambiguous. The detec-tion of additional -/-lines from 70Cu in the experimentof TafT et al. is a consequence of the reduced inter-ference from other Cu-isotopes achieved by the use ofan enriched target.

OTHER APPLICATIONS OF THESISAK TECHNIQUE

Although the SISAK technique was originallydeveloped for chemical isolation and identification ofshort-lived nuclides, it has properties for a potentialapplication in other fields.

The SISAK technique permits studies of chemicalspecies with half-lives down to about 5 sec, providedthese species distribute in two-phase liquid media. Ofinterest are the chemical properties of radionuclidesproduced in nuclear reactions (transformations) invarious chemical environments as well as the chemicalproperties of radicals formed in radiation reactions.Radiolysis products in condensed media are poorlyknown and might well be investigated by the rapidtwo-phase SISAK system.

The on-line technique makes it possible to con-tinuously isolate short-lived radiotracers for chemical,medical or industrial applications over quite extendedperiods. Thus, e.g.28AI (half-life 2-2 min) may becontinuously produced and its wet chemistry in-vestigated by the use of the SISAK technique. Similarly13N (half-life 10 min) may be produced and throughthe two-phase system—bound into a suitable com-pound for medical purposes.

Acknowledgements—The authors are indebted to theircolleagues at the Departments of Nuclear Chemistry inGöteborg and Oslo for valuable suggestions and assistanceThe authors also express their gratitude to Professor A. C.Pappas for his support during the initial stage of thiscollaboration. Mr. H. Persson and Mr. L Bitsvik wirreresponsible for the mechanical construction work a idMrs. E. Norman, Miss R. Johansson and Mrs. E Jomarassisted in some of the experiments. Mrs. A. Birch Aune.Mrs. M. Carlson and Miss C. Hard af Segerstad typed ourmanuscript in a deserving way. The Swedish Atomic Re-search Council and the Norwegian Research Council forScience and the Humanities gave financial support.

REFERENCES

1. I. Bergström, Nuct. Instr. Methods $3. 116 (1966)2. W. E. Talbert, Proc. Int. Conf. on the Properties ol

Nuclei Far From the Region of Beta-Stability, Leysin1970, CERN-Report 70-30, p. 109(1970).

3. H. D. Schüssler, W. Grimm, M. Weber. V. Tharum.H. O. Denschlag and G. Herrmann, Nuel. Imtr.Methods 73, 125(1969).

4. G. Herrmann, Rapid radiochemical separations. Re-view paper presented at the Radiochemical Seminar.Dubna, October 1970.

5. H. Reinhardt and J. Rydberg, Ada them, scund. 23,2773(1969).

rSISAK a technique for (radio)chemical separations 2401

6 J Rydberg. H. Reinhardt and J O. Liljen/in. IonExihange and Soli: E.Mr.3. Ill (1973).

7. P. O Aronsson. E Ehn and J. Rydberg. Plus. Rei:Leu 25.590(1970).

8. P O. Aronsson. G. Skarnemark and M. Skarestad.J. morg. nut I. Chem. 36. 1689 (1974).

9. C. Andersson. S. O. Andersson. J. O. Liljenzin. H.Reinhardt and J. Rydberg, Ada chem. scand. 23, 2781(1969).

10. P. O. Aronsson. diss. Chalmers Univ. of Technology.Göteborg. To be submitted.

11. W. Eckhardt. G. Herrmann and H. D. Schlüssler.Z. anatyt. Chem. 226. 71 (1967)

12. W. M. Latimer, Oxidation Potentials, 2nd Edn, pp. 341 -342. Prenlice-Hall. New York (1952).

13. R. Gunnink, Identification and determination ofgamma-emitters by computer analysis of Ge(Li)-spectra, UCID-15140, 1967.

14. J. T. Roulti and S. G. Prussin, Nucl. te/r. Methods 72,125(1969).

15. E. C. Campbell and F. Nelson. J. tnorg. nucl. Chem. 3,233(1956).

16. J. Blachot and R. de Tourreil, J. radioanal. Chem. II,351 (1972).

17. P. A Seeger. Nucl. Phys. 25. 1 (1961).18. H. Bakhru and S. K. Mukherjee. Nad. Phvs. 52. 125

(1965).19. D. G. Gardner, Nm-I. Phys. 29. 373 (1962).20. H. Neuerth and H. Pollehn, Tables of Cross Sections of

Nuclear Reactions with Neutrons in the 14-15 MeVEnergy Range. Euratom Report I22e, 1963.

21. H. Stephen and T. Stephen. Soliibiliiie\ of hwrgtwti iintlOrganic Components, p. 531. Pcrgamon Preis. Oxford(1963).

22. B. Sundvoll, Department of Nuclear Chemistry. Univer-sity of Oslo, 1972. Unpublished.

23. C. M. Lederer. 3. M. Hollander and 1. Perlman. Tableof Isotopes, 6th Edn. John Wiley, New York (1967).

24. H. K. Carter, J. H. Hamilton and J. i. Pinajjian. Phys.««•.178,1743(1969).

25. T. E. Ward.H. IhochiandJ. L. Meason. Phv, Ret: 188.1802(1969).

26. H. Singh, V. K. Tikku, B. Sethi and S. K. Mukherjee.Nucl. Phys. A174,426 (1971 ).

27. J. L. Meason and P. K. Kuroda, Phys. Rei: 138, 1390(1965).

28. C. M. Taff, B. K. S. Koene and J. van Klinken. Nucl.Phys. M64, 565(1971).

29. R. R. Swanson, U.S. Pat. 3224873, 1965.30. R. R. Swanson and I. L. Drobnich, Trans. Soc Min.

Engrs AIME, Denier, Colorado (x970).31. D. W. Agers, I. E. House, R. R. Swanson and I. L.

Drobnich, Trans. Soc. Mm. Engrs AIME. DenierColorado (1966).

32. D. L. Swindle, N. A. Morcos, T. E. Ward and J. L.Meason, Nucl. Phys. A185, 561 (1972).

33. A. Szalay and K. Jost, Radiochem. Radioanal. Lett 15.57(1973).

Afterthe completion of this manuscript, further informationon 68Cu and 68"Cu has been published by Swindle et a/.[32].Assignments of the 7oCu-isomers has been made by Szala)and Jost[33].

rINORG. NUCL. CHEM. LETTERS Vol. 10. pp. «9-504. 1974. Per|amon Preii. Printed in Gieal Brunn.

THE HALF-LIFE OF 150Ce OBTAINED BY SISAK TECHNIQUE

P.O.Aronsson and G.Skarnemark

Department of Nuclear Chemistry, Chalmers Univers i ty o f Technology,

Fack, S-402 20 Göteborg 5, Sweden

and

M.Skarestad

Department of Nuclear Chemistry, University of Oslo, Oslo 3, Norway

(Received 4 Maren 1974)

The half-life of the previously unidentified nuclide IS^Ce has, byusing the fast chemical separation technique SISAK, been determined to be4.0 ± 0.6 s. In the same way, the half-life of its daughter product lS0has been determined to be 6.2 ± 0.2 s.

INTRODUCTION

In a previous paper (1 ) , we presented ind i rect evidence fo r the

iso la t i on of Ce wi th a presumed h a l f - l i f e of some 5"'O s. This evidence

was based on the presence of Pr (E = 131 keV) in a chemically150separated Ce f rac t i on . The decay-curve presented for Pr also showed an

increased h a l f - l i f e , which could thus be due to the growth from Ce.

In the fo l lowing we wi11 discuss fur ther th is evidence by including

growth- and decay-measurements of the 131 keV Y- l ine in various chemical

f r ac t i ons . The study is based on the SISAK technique and the two-detector-

delay (TDD) method as described in re fs . (2 , ' t ) .

EXPERIMENTAL

Irradiations and chemical separations. The irradiations were performed

with a }k MeV neutron generator. The target consisted of a UO^O^ complex

-irisorbed on a Dowex 1x8 resin, from which fission product? were

continuously eluted (1,2). The following three type' of experiments (see

also ref. (1)) were performed, using the SISAK technique:

( i ) A Ce(IV) fraction is separated ac:ording to ii-.« experimental

©

001M IMHNO3INH^Sq m M ^

005 MQ1005M

©0.3 M HDEHP IN KEROSENE

»LnftiD.Ng Ce(IV),Ng

uo2so4-COMPLEX0ND0WEX-1

P

1 M LACTICACID

FIGURE 1.

150rFlow diagrams showing the separation of '3aCe from fission products, (a) corresponds to experiment (i), (b) to ex-

periment (ii) and (iii). n • neutron beam, FP « fission products, Ng - noble gases, CI, CZ, C3 « mixer-centrifugal

separator units, DI, D2 - detectors. TDD - flow cells and delay lines for two-detector delay measurements.

J

rVol. 10, No, 6 HALF-LIFE OF ls0Ce

set up schematically shown in Fig. la. Ce(IV) is extracted from an

oxidizing solution (1 M HNO,, 0.1 M h^SO, and 0.05 M K2Cr20_) into an

organic phase consisting of 0.3 M HDEHP in kerosene. The phases are

separated in CI and the organic phase is then washed by a new contact with

an oxidizing solution. The TDD measurements are carried out on the organic

phase leaving C2.

A good separation between Ce and Pr is obtained; the separation factor

being of the order 10 with respect to primarily extracted Pr.

(ii) Trivalent lanthanides are extracted from a nitric acid solution

(pH ~ I.1») into an organic phase consisting of 0.3 M H0EHP in kerosene

(see Fig. 1b). La, Pr and some Ce are then stripped in C2 by an aqueous

phase consisting of 1 H HUOy 0.1 M »2%0k and °- ° 5 M K2Cr2°7- The TDD

measurements are now carried out on the aqueous phase leaving C2, where

the Ce and Pr are present in stripping yields of about 30 % and 95 %,

respectively.

(iii) The same chemical system as in (ii), but now the aqueous phase

leaving C2 passes through an HDEHP-PVC column prior to the TDD measure-

ments. The HDEHP-PVC column efficiently removes the Ce remaining in the

aqueous phase.

Measuring equipment and data evaluation. The detection of Y-rays was

carried out using the Ge(Li)-detector system described in ref. (2). The

photo peak areas were determined by the program GAMANL (3).

(t has been shown in réf. W that the TOO method is convenient for

half-life determination of nuclides with T ^ 2 below some 50 s. However, if

growing-in occurs, the equation for Q as defined in ref. (2) becomes more

complicated. In general, however, the equation may be expressed in the

fol lowing way:

Q = Ae~V + Be'V

where the symbols denote

Q = ratio between the counting rates of detectors 2 and 1

X. = decay constant of the parent

X2 = decay constant of the daughter

t = T ] + T = delay time, where

Tj = hold up time in detector cell 1

T = hold up time in the variable delay tube

A and B are constants when the following assumptions are justified: (see

also ref. C O )

HALF-LIFE OF 1 Ä C r Vol. 10. No. 6

(a) a constant flow rate with Reynold's number > 2300 throughout the

experiment

(b) a constant ratio between the disintegration rates of parent and

daughter at the entrance of detector cell 1.

These assumptions are justified to a good approximation in our present

experiments.

A plot of 0. versus .he delay time thus gives a curve which may be

resolved into two components giving the decay constants X, and *2>

respecti vely.

RESULTS

In Fig. 2a, the experimental Q-values obtained for the 131 keVy-line

are shown as a function of the delay time in experiment (i). A growth of

the photo peak is demonstrated, and a numeric resolution of the curve

based on the formula for d and the minimization program MINUIT (5) gives

the half-lives for 1 5 0Ce and I 5 0Pr shown in Table 1.

In Fig. 2b, corresponding to experiment (ii), there is still a

significant growth of Pr, although the number of daughter atoms present

at the entrance c detector cell 1 is substantially higher in this case.

The half-lives otudined are given in Table 1.

TABLE 1.

150rSummary of Ha If-lives of Ce and15(W.

Nuclide

1 5 0Ce

Mean value

150pr

Mean value

Tl/2

•*•13

3.94

it.O

6.00

6.50

6.22

6.2

t

t

t

i

t

i

±

0

0

0

0

0

0

0

s

.95

.86

.6

• 5 7

.15

.39

.2

Exp. Fig.

6.1 ±0.3 ref. (6)

2a

2b

2a

2b

2c

1 0 F

0.1

0.01

1 1 1

10 20 30

I 1 I

10 20 30

Delaytime. s

0 10 20 30 40 50 60 700.01

FIGURE 2 .

Growth and decay of the 131 keV peak of '^0Pr, showing the presence of Ce. (7) and (IT) , corresponding to

experiments (i) and (li) respectively, show the decay in fractions containing both Ce and grown-ln Pr. fc}

shows the decay of the pure Pr fraction of experiment (ill).

HALF-LIFE OF Vol. 10. No. 6

In Fig. 2c, the decay-curve for the 131 keV photo peak with no

contribution from Ce (cf. experiment (Hi)) is shown. The half-life of

6.2 ± 0.4 s is obtained after subtraction of the observed background

activity. This background activity can be neglected in experiments (i) and

(ii), since the total activity of the 131 keV peak is higher, due to a

shorter transport time from the target to the detector.

in conclusion, we find that the mean value of 6.2 * 0.2 s obtained

for Pr is in good agreement with the value of 6.1 * 0.3 s reported by

Ward et al. (6). The half-life of 4.0 ± 0.6 s observed for 1 5 0Ce fits in

well with the systematic trend in the half-lives of other neutron-rich

Ce isotopes.

ACKNOWLEDGEMENTS

The authors are indebted to the Swedish Atomic Research Council and

the Norwegian Research Council for Science and Humanities for financial

support.

REFERENCES

1. P.O.ARONSSON, G.SKARNEMARK and M.SKARESTAD, Short-lived Isotopes of

Lanthanum, Cerium and Praseodymium Studied by SISAK technique, J. inorg.

nucl. Chem. In press.

2. P.O.ARONSSON, B.E.JOHANSSON, J.RYDBERG, G.SKARNEMARK, J.ALSTAD, B.

B E K S E R S E N , E.KVÂLE and M.SKARESTAD, SISAK - A New Technique for Rapid,

Continuous, (Radio)chemical Separations, J. inorg. nucl. Chem. In press.

3. R.GUNNINK, Identification and determination of gamma-emitters by

computer analysis of Ge(Li)-spectra. UCID-15140 (1967).

4. P.O.ARONSSON, Dissertation, Chalmers Univ. of Technology, to be

submitted 1974.

5. F.JAMES and M.ROOS, CERN Computer, 6000 Series Program Library, Long-

Write- Up D506, D516 (1971).

6. T.E.WARD, N.A.MORCOS and P.K.KURODA, Phys. Rev. £2, 2410 (1970).

rIG. NUCL CUKM. LETH-RS Vol. 10. pp, 753-762. 1974. M p m o n Picis. Primed in C.rcil Bnlain.

DEWY CHARACTERISTICS OF SOME NEUTRON-RICH LANTHANIDE NUCLIDES OBTAINED BY

SISAK TECHNIQUE.

P.O-Aronsson and G.Skarnemark

Department of Nuclear Chemistry, Chalmers University of Technology, Fack,

S-<402 20 Göteborg 5, Sweden

and

E.Kvâle and M.Skarestad

Department of Nuclear Chemistry, University of Oslo, Oslo 3, Norway.

(Received 6 May 1974)

YT coincidence measurements on short-lived, neutron-rich lanthanidenuclides have been performed using the fast chemical separation systemSISAK. The results include the assignment of 11 y-rays to ^47Ce and 6 tol^^Ce, decaying with mean half-lives of S6.4±1.2 s and 48. 1±1. 1 s, respec-tively. Several previously unknown ~f-rays are reported for I44La as well asfor 148pr_ Partial decay schemes are proposed for ^4^La and ^eLa, and theenergy levels are discussed in terms of ground state rotational levels.

INTRODUCTION

The properties of nuclides in the shape transition region N » 82-90

have recently become a subject of great interest (1) . The abrupt shape

transition occurring in the 5m-Gd region, where nuclei with N - 88 have

vibrational-type spectra, and nuclei with N = 90 have rotational-type

spectra is well known (2). Outside this region, experiments show a smoother

behaviour of the parameters associated with deformation, and large devi-

ations from both the harmonic vibrator model and the rigid rotor model are

observed (3,1*).

To contribute necessary experimental data regarding such transitional

nuclei, we started a series of experiments on heavy La, Ce and Pr nuclides

produced in neutron-induced fission of uranium (5 ,6) . The present paper

deals with some of the decay characteristics obtained on the basis of Y"Y

coincidence measurements.

The chemical separation of the short-lived species is fac i l i ta ted by

0.3 M HDEHP IN KEROSENE

uo2so4COMPLEXONDOWEXI

001 M 1 M HNO3 1 M HNO3 1 M HNO3(NH.LSO. 01MH2SO4 0.05 M H ^ 01M H ^

CHÄMK^Cr^ 0.02 M SAA 005 M

03 M HDEHP IN KEROSENE

uo2so4COMPLEXONDOWEXI

001M pH 1.4 1M HNO301 M HSO

FIGURE 1.

Flow diagrams showing the systems used for y-y coincidence measurements. ( J ) is the system used for Ce, (b) the

one used for La and Pr. n = neutron beam, FP = f i s s i on products, Ng = noble gases, SAA = sulfamic ac id , C1.C2 =

mixer-centr i fugal separator un i ts , E - ext ract ion column (HDEHP on PVC beads), I = ionic exchange column (Dowex

50 W x 1*, 50-100 mesh), 01,02 = Ge(Li)-detectors.

\

Vol. 10, No. 9 NEUTRON-RICH LANTHANIDI: NUCLIDES

C

the fast, on-line chemical separation system SISAK, described in ref. (7)-

EXPERI MENTAL

Irradiations and chemical separations. All irradiations were performed

with a 111 MeV neutron generator. The target consisted of a UO^SO^ complex

adsorbed on a Dowex 1x8 resin, from which fission products were continu-

ously eluted (5).

The chemical separation system used for the isolation of Ce and

Ce is shown schematically in Fig. la. After elution from the target,

Ce(lll) is oxidized to Ce(IV) by making the solution 1 M in HNOj, 0. I H in

H,S0. and 0.05 M in K^Cr-O- (below referred to as an oxidizing solution).

From the oxidizing solution. Ce is extracted (in CI) with 0.3 H HDEHP in

kerosene. In C2, Ce is reduced and stripped by 1 H HNO,, 0.05 H K ^ O ^ and

0.02 M sulfamic acid; then H,S0t and K,Cr,07 are added in the proper

quantities to oxidize Ce, which is then adsorbed on a HDEHP/PVC column,

serving as a source for the Y"Y coincidence measurements.

For the isolation of short-lived La and Pr nuclides, a separation

system corresponding to the flow sheet shown in Fig. 1b was used. The

eluate from the target is now made acidic (pH = \.k) by adding HNO,, then

the trivalent lanthanides are extracted (in Cl) with 0.3 M HDEHP in kero-

sene. In C2, trivalent lanthanides (mainly La and Pr) are stripped by an

oxidizing solution leaving Ce in the organic phase. The y~T coincidence1 1»

measurements are then carried out either on a Dowex-50 column ( La,

Pr) or directly on the streaming liquid ( La). The chemical identity

of the Pr y-rays was proven by stripping Pr from a Ce fraction as

described in (5).

Measuring equipment and data evaluation. The y-y coincidence system

consisted of two Ge(Li)-detectors (relative efficiencies 7-8 and 13 %) and

standard electronic equipment interfaced to a PDP-7 computer. The time

resolution of the system (FWHM) was 15 ns and the gate window 30 ns. The

system allowed correction for random coincidences. The evaluation of y-ray

energies, photo peak areas and intensities was carried out with the program

GAMANL (9). The half-lives were determined numerically with the program

MINUIT (10).

RESULTS AND DISCUSSION

Ce and Ce. In a previous paper (5), we have reported several y-

rays decaying with half-lives in the region of 1(0-60 s. These y-rays were

attributed to Ce and Ce, but the small difference in the half-lives

r756 NEUTRON-RICH LANTHANIDE NUCUDFS VoL 10, No. 9

of these two nuclides made further, individual assignment difficult. To

overcome this difficulty, we performed Y"Y coincidence measurements on a

Ce source, and the results are summarized in Tables 1 and 2.

Based on the decay of individual y-rays, the half-lives of Ce and

Ce were determined to be 56.'»1.2 s and 1)8.1*1.1 s, respectively.

TABLE 1.

Energy, keV

90-5

105.4

121.4

196.0

269.5

325-2

360.0

374.7

A67.6

581. B

606. 1

Energy, keV

99-2

117.3

218.6

292.6

390.4

452-5

Decay

T 1 / 2 , s

comp1e x

55*5

53*3

50±4

58*3

59±3

55*5

59*3

55±5

60+3

60±4

Decay

T 1 / 2 ' S

55*3

45*2

56*5

45*5

1)2*5

50*5

characteristics of

I Y . *

75

9

37

50

100

25

816

18

20

9

TABLE 2.

characteristics of

1 . *

51

9

30

100

7

27

""Ce.

Gating energies, keV

0

0

X

0

0

I l l 8Ce.

Gatii

<n

0

0

X

r g

0

0

X

0

vOOJ

X

X

0

0

rig energies, keV0 0

X X

0

Footnote: In Tables 1-4 the following symbols are used under "Gating ener-gies": X=strong coincidence ( 50 counts in the photo peak), 0=weak coin-cidence (10-50 counts in the photo peak). The uncertainty in the energydetermination is *0.2 keV. The relative uncertainty in I is 5 15 %•

rVoL 10. N a 9 NEUTRON-RICH LANTHANIDE NUCLIDES

144 146 144

La and ha. Partial level schemes of the even-even nuclides Ce

and Ce have been deduced on the basis of the Y~Y coincidence study of

the La/Pr-fraction. A central question arising is whether any of the ground

state rotational levels are populated by the ß'-decay, and in the discussion

of this we would like to present the following considerations:

(i) The partial decay scheme proposed for I l s La in Fig. 2a is in

excellent fit with the energy levels of the 2 +, 4 and 6 states in Ce

as predicted by the variable-moment-of-inertia (VMI) model of Scharff-

Goldhaber et al (8). This model, describing each nucleus by two adjustable

parameters, 9 (ground state moment of inertia) and a (softness parameter),

is known to provide remarkably good fits to the ground-state band rotational

energies of even-even nuclei as well as to those of higher bands, such as

the K = 2 (y-vibrational) bands.

The fitting procedure used for computation of the level energies E( is

in the actual cases based on the equations given by Scharff-Goldhaber et al

(8) along with the minimization program MINUIT (10). The values of 9 and

o in the case of |i|6Ce are 3 = 0.0077 kev"1 and a = 0.1*9.° 144

A comparison between the energy levels in Ce and the VHI-predicted

2*, 4 and 6 + states is presented in Fig. 2b. In this case, however, the

agreement is less striking although the deviation is still within reason-

able limits (± 2 %). The values of 3 = 0.0017 kev'1 and o = 34.9 indicate144 146

that Ce is less deformed in the ground state than Ce, which should144be expected since Ce is situated closer to the N » 82 shell.144 146(ii) The energy levels in Ce and Ce which may be associated with

2 +, 4 + and 6 + states show good agreement with the behaviour established

for E.+/E2+ and E,+/E,+ ratios as shown in Fig. 3 and 4. The Hal Imann curve

(12) presented in Fig. 4 is based on the observation that for even-even

nuclei with widely differing Z, N and E2+ values, the energy ratio E,+/E,+

plotted against E.+/E,+ lies on a 'universal1 curve. This observation is

now supported by a Irrge amount of experimental data (4,8).144 146(iii) An interpret .tion of the levels in Ce and Ce in terms of

ground-state rotational levels is consistent with the y-y coincidence

results as well as with the fact that no transitions which would involve

AI > 4 have been obser-jd in y-ray single spectra (relative intensities

less than 1 %).

Thus it appears rather conclusive that the decay of 11 s La and 40 s144 + + + 146 144

La populate the 2 , 4 and 6 levels in Ce and Ce, respectively.146In the case of Ce, this conclusion is in agreement with the findings of

NEUTRON-RICH LANTHANIDE NUCUDES V o l l a No. 9

146La \T1/2=11s (S)

Qa=5200 këU3 cale (11)

O>

g s P 1171.1

668.2-Sm 258.5

6*

4*

2*0*

1171

668

258

146Ce

144La \T1/2=4OS(5)

Qß=4500 keV Vp cale (11) ß"

6* 1539

923

0*144,

FIGURE 2.

Partial decay schemes of La and La. The energies are in keVand relative intensities (in %) are given in brackets. Calculatedenergies of the 2 +, k* and 6+ states, based on the VMI model, areshown on the right.

rVoL 10, No. 9 NEUTRON-RICH LANTHANIDE NUCLIDES

Cheifetz et al (13), who measured prompt K X-rays and/or y-rays in coinci-

dence with pairs of fission fragments produced in a 2 5 2Cf source. In their

experiments, however, only the 2 + -. 0 + transition in ''"''ce was observed.

It should be emphasized that only the most prominent levels are in-

cluded in the present partial decay scheme of ^ L a . In fact, this nuclide

seems to be rather y-rich, and several low intensity Y-rays have been ob-

served both in y-Y coincidence spectra and Y-single spectra. Some of these

Y-rays are reported also in a recent publication by Ohyoshi et al ("i). The

data so far available on La are listed in Table 3.

2.4

64 66

FIGURE 3.

Systematic behaviour of the ratio E ^ / E , * as a function of theproton nurober in the N=86-92 region. The data represented asopen squares are from the current experimental results. Thef i l l e d circles indicate data from refs. (3.U).

E6+/E2+ 5.0

2.4 2.6 2.8 3.0 3.2

FIGURE lt.

Plot of the energy ratio Et+/E,+ versusbased on data from refs. (4,8) and the icurrent experimental results.

The line drawn isopen squares indicate the

J

760 NEUTRON-RICH LANTHANIDE NUCUDES Vol 10. No. 9

Ene rgy, keV

TABLE 3.

Decay characteristics of

V *144La.

T1/2- s Gating energies, keVO% LA t—

? •£ 8139.5

165.3226.831't.O

397.3<i3l.5S'il. 1

585.0

705.'»

735.3

Ö44.9

951. it

129!.. 9

complex

39*4

-

-

40*2

31*7

<<0±2

40*8

-

37±5

35*3

-

38*5

7.0

14

2.1

2.4

100

5-742

I t

5.0

12

29

7.5

8.9

X

X

y

X

0 X

0 X

0

0

X

0

X

X

X

X

X

X

X

0

X

X X X

X

X

X

X

Pr. The Y ~ Y coincidence measurements in combination with half-life

determinations revealed the existence of several low intensity y-rays,

decaying with half-lives between 2 and 3 minutes. The assignment of these

Y-rays to Pr is based primarily on the fact that these lines are present

in the Pr fraction stripped from Ce even though the delay time (5) between

the target and the first extraction stage is increased so that no Ce is

extracted. This excludes the possibility of these lines belonging to Pr.

A few of these y-lines were also reported by Ohyoshi et al in a recent

publication (15)- The mean half-life of Pr was determined, from the

strongest peaks at 300.1, 701.3 and 1028.8 keV, to be 2.2+0.1 min. Our

results for Pr are listed in Table 4.

ACKNOWLEDGEMENTS

We are indebted to Dr. F. Ingebritsen for his kind assistance in setting

up the coincidence equipment, to Miss. C. Hard af Segerstad for the

admirable typing of our r. nuscript and to the Swedish Atomic Research

Council and the Norwegian Research Council for Science and Humanities for

fincancial support.

Vol 10. No. 9 NEUTRON-RICH LANTHANIDE NUCUDES

Energy, keV

157.5

256.4

300.1

493.1

518.1

541.9

617.4

701.3

874.3

1001.4

1029.0

1365-6

TABLE 4.

Oecay characteristics of Pr.

T , / r min

1.1*0.3

3.2±0.7

2.3*0.1

2.6*0.1

1.3*0.9

2.8±2.4

2.0±0.5

2. 1±0.1

2.8±0.2

1.5*0.1

2.2*0.4

3.2*0.4

REFERENCES.

V *

1.5

5.0

100.0

1.7

3.5

1.1

4.7

9.0

6.9

2.0

8.9

8.1

Gating energy, keV

300

X

0

0

X

X

1. K.KUMAR in Proo. Int. Conf. on the Properties of lluolei far from the

Region of Beta-Stability, Leysin M70, CERN-Report 70-30, p. 779 (1970).

2. G.SCHARFF-GOLOHABER and J.WENESER, Phys. Rev. 9 8 , 212 (1955 ) .

3. F.S.STEPHENS, D.WARO and J.O.NEWTON, </. Phys. Soo. Japan Ik, 160 (1968 ) .

k. E.CHEIFETZ, R.C.JARED, S.G.THOMSON and J.B.VILHEIMY, in Proa. Int. Conf.

on the Properties of Nuclei far from the Region of Beta-Stability,

Leysin 1970, CERN-Report 70-30, p . 883 (1970).

5. P.O.AR0NSS0N, G.SKARNEMARK and M.SKARESTAD, J. ivorg. nucl. Chem. 3b

( 1 9 7 4 ) . In press.

6 . P.O.ARONSSON, G. SKARNEHARK and M.SKARESTAD, Inorg. nucl. Chen. Lett.

JO ( 1 9 7 4 ) . In p ress .

7. P.O.ARONSSON, B.E.JOHANSSON, J.RYDBERG, G.SKARNEMARK, J.ALSTAD, B.

BERGERSEN, E.KVÂLE and M.SKARESTAD, J. inorg. nucl. Chem. 36 ( 1 9 7 4 ) .

In press .

8. M.A.J.MAR I SCOTT I , G.SCHARFF-GOLDHABER and B.BUCK, Phys. Rev. J]S, 1864

(1969 ) .

9. R.GUNNINK, UCID-15140 ( 1 9 6 7 ) .

10. F.JAMES and M. ROOS, CERN Computer, 6000 Series Program Library, Long-

Write-Up 0506, 0516 ( 1 9 7 1 ) .

rNKUTRON-RICH LANTHANIDF. NUCLIDES Vol. 10. No. 9

11. P.A.SEEGER in Proa. Int. Conf. on the Properties of Nuclei far from

the fiegion of Beta-Stability, Leysin 1970, CERN-Report 70-30, p. 217

(1970).

12. C.A.MALLMANN, Phya. Rev. Lett. 2, 507 (1959).

13. E.CHEIFETZ, J.B.WILHE1.MY, R.C.JARED and S.G.THOMSON, Phys. Rev. kC,

1913 (1971).

14. A.OHYOSHI, E.OHYOSHI, T.TAMAI, H.TAKEHI and M.SHINAGAWA, J. nuol. Soi.

Techn. 9, 658 (1972).

15. E.OHYOSHI, A.OHYOSHI, T.TAMAI, H.TAKEMI and M.SHINAGAWA, J. nuol. Soi.

Techn. 10, 101 (1973).

rINORG. NUCL. CHUM. LGTTMtS V o l I I . pp. 729-735. 1975. Ferpmon neu. Primed in Great Britain

THE COMBINATION OF THE GAS JET RECOIL TECHNIQUE WITH THE

FAST CHEMICAL ON-LINE SEPARATION SYSTEM SISAK

N. Trautmann+, P.O. Aronsson*, T. Björnstad++, N. Kaffrell+,

E. Kvâle++, M. Skarestad++, G. Skarnerâark* and E. Stender*

The SISAK Collaboration

(Received 10 June 197S)

ABSTRACT

The fast chemical on-line separation system SISAK has been

connected to a gas jet recoil transport arrangement. Pure

fractions of short-lived La, Ce and Pr nuclides have been

isolated from complex mixtures of reaction products obtained

by thermal-neutron-induced fission of 235U.

INTRODUCTION

The Gas Jet Recoil Transport (GJRT) system has been used

successfully to transport short-lived reaction products from

a highly radioactive irradiation site to a low background area

Institut für Kernchemie, Universität Mainz, D-6500 Mainz,Germany

Department of Nuclear Chemistry, Chalmers University ofTechnology, Fack, S-40220 Göteborg 5, Sweden

Department of Nuclear Chemistry, University of Oslo, Oslo 3,Norway

Fast Chemical On-line Separation System SISAK Vol 11. No. II

EXPERIMENTAL

This paper will mainly concentrate on the Ce system although

chemical systems for La and Pr were tested and found to work

properly. The whole system shown in Fig. 1 consists of three

main parts: the GJRT arrangement, the mixing-degassing unit

and the chemical separation system.

The GJRT system. The target, consisting of 450 vg 235Ucovered with a 400 ug/cm Al layer, is situated in a recoil

chamber of about 12 cm volume. The recoil chamber is placed

(1-5). This technique has not only been applied for charged-

particle reaction products (1,4) but also for radioactive

products resulting from neutron-induced fission (3), spontaneous

fission (2,5) and radioactive decay (6).

The nuclides recoiling out of the irradiated target are

thermalized in a gas, usually He mixed with some organic

compounds to form the necessary clusters. The products are

then transported through a capillary, the length of which may

be several meters.

In most of these experiments, the nuclides transported by

the jet have been collected by impinging or. a collector plate

(1-5). Only in a few cases, attempts have been made to separate

individual elements from product mixtures by combining the

jet system with chemical procedures (4,6-9). So far, no severe

efforts have been undertaken to utilize the GJRT technique in

combination with a continuous chemical separation system for

the selective on-line isolation of elements from complex mixtures

of reaction products, e.g., fission products.

In this paper, we will report on the connection of the fast,

continuous radiochemical separation system SISAK (10,11) to a

GJRT arrangement and its application to the separation of short-

lived fission products. The chemical system previously used for

studies on neutron-rich isotopes of La, Ce and Pr (12,13,14) as

well as the gas jet have been modified to enable the successful

achievement. In the -y-ray spectra of the La, Ce arid Fr fractions

nuclides with half-lives down to fi s have be^n ob'.°rved.

rVol. 11, No. II Tasl Chemical Onl ine Separation System SISAK

0 3H «DEHP m STiFllsol !

1HH«,0 (HH,SO,

IHHNDj 1H H,SD4

0 IN H,SOt O.ObH SI«0 O5H K,Cr,[l. 0B5HH,0.

FIG. 1

Flow diagram showing the system for the isolation of Cenuclides. M:gas-liquid mixer, Dg:degassing unit, FP:fissionproducts, Cl, C2 , C3:mixer-centrifugal separator units, E:HDEHP/PVC column, DO-D7:Ge(Li) detector positions, YO-YM:Cerecovery yields. YO > 80 % (calculated), Yl > 90 % (measured),Y2 -v 80 % (measured), Y3 > 30 % (measured), Y"+ 55 % (measured)

in one of the beam holes close to the core of the Mainz TR_GA11 — 2 —1reactor, having a thermal neutron flux of about 10 n cm s

The fission products recoiling out of the target are

thermalized in a 1 :1. H mixture of C-H^ and N~ , the former

substance serving as a cluster producer (15,16). The gas pressure

in the recoil chamber is kept at 1500 torr, and the gas flow3 —1rate at some 20 cm s . When thermalized, the fission products

are transported to the mixing-degassing unit via a 7 m poly-

ethylene capillary (inner diameter 1 mm). The transport time

from the target to the erui of the capillary has been measured

to amount to about 1 s.

The mixing-degassing unit. After the transport through the

capillary, the gas is mixed in a static mixer with the first

aqueous phased . 25 M HNOj for Ce and Pr or, if studying La, a

HNO3 solution of pH 1.4 ). The temperature of this solution is

kept at about 90°C, since a strong influence of the temperature

rFail Chemical On-line Sepjfaiiun System SISAK Vol I I , Nu. II

on the fission product dissolution was noticed.

The gas-liquid mixture is then fed into a degassing unit,

simply consisting of a coned funnel with a tangential inlet.

In this funnel, the C^tt^-U^ mixture is swept off together

with more than 95 % of the noble gas activity.

The chemical separation system. The degassed liquid is

pumped to a second static mixer, where it is contacted with

the first organic phase (2 M HDEHP in Shellsol T). In this step,

most of the Y, Zr, Nb and Ko as well as part of the Br and I

are extracted into the organic phase. The phases are then

separated in the first H-centrifuge C 1 (17).

The Ce, remaining in the aqueous phase from C 1 together

with the majority of the fission products, is oxidized to the

tetravalent state by adding HNO3, H^SO^ and K2Cr?07. It is

then extracted almost quantitatively (in C 2) into the second

organic phase (0.3 M HDEHP in Shellsol T). In C j, Ce is

stripped from the organic phase bv using HjSO^, sulfamic acid

(SAA) and H^O^. After re-oxidation achieved by adding the proper

amount of K,Cr2O7, the Ce is adsorbed on an HDEHP/PVC column,

as described i" ref. (13).

Measuring equipment and data evaluation. The measuri-ig system

consisted of a Ge(Li )-detector (efficiency 6.1» %) connee'ed to

a 4 K multichannel analyzer. The data obtained were stored on

magnetic tapes and evaluated with computer programs.


As mentioned above the dissolution of the fission products

is influenced by the temperature of the first aqueous solution.

Generally, a higher temperature leads to an increased yield of

the desired element and a better decontamination from interfering

elements. This is probably due to a partial extraction of the

undestroyed clusters into the organic phases at ]ow temperatures.

This assumption is supported by the fact that in the cerium

fraction the amount of the trivalent lanthanides decreased

rapidly with increasing temperature of the first aqueous solution,

whereas the yield of Ce is increasing. At tempera'-ui es of about

90°C, the strong 397 keV peak of lltl4La, which served as a monitor

rVol. 11, No. 11 Fast Che«licllOii*iie Separation System SISAK

for trivalent lanthanides, disappeared in the Ce fraction.

On-line measurements were performed in the different positions

indicated in Fig. 1. The column E as well as the aqueous phase

leaving the unit C 3, showed almost no other activities than

those from Ce nuclides and, to some extent, the grown-in Pr

daughters. A y-ray spectrum of the Ce fraction on the column E

is shown in Fig. 2. In this spectrum, the peaks of Si - Ce

ar.dt8-s 1U8Ce can be identified, as well as those of the longer-

lived isotopes 3-min ll*5Ce and 13-min ll*6Ce (12,in).

The efficiency of the gas-liquid transfer of Ce was determined

in the following way: the activity of the strong 317 keV peak

of 146Ce was measured in a Ce sample collected on the HDEHP/PVC

column and compared to a "direct catch" measurement, obtained

by passing the gas through a fiber glass filter. The ratio column/

direct catch was calculated to be 0.1. From this value and the

measured chemical yields indicated in Fig. 1., the Ce dissolution

yield in the gas-liquid mixer can be calculated to be = 80 %.

By using the chemical separation procedures for Ls and Pr,

strong sources of 11-s La and of 5-s Pr, respectively,

were produced.

The results obtained show that the combination of the GJRT

technique with a fast, continuous chemical separation system like

SISAK enableson-lins investigations of specific short-lived

fission products. Detailed spectroscopic studies by >-•y-

coincidence and even y-y-angular correlation measurements

should be possible. In addition to studies of fission products,the

described system can be used for the investigation of nuclei

formed in spallation and heavy-ion-induced reactions.

ACKNOWLEDGEMENTS

The authors express their gratitude to Professors

G. Herrmann, A.C. Pappus änd J. Rydberg for their interest

in this work. We wish to thank Mr. R. Heimann for his help

and the staff_ of the Mainz TPIGA reactor for numerous

irradiations. Financial support from the Swedish Atomic

Research Council, the Norwegian Research Council for Science

and the Humanities and the Bundesministerium für Forschung

und Technologie is gratefully acknowledged.

r734 Fist Chemicil On-taM Sepnilnn System SISAK V o l 11,No 11

UDO IN»• Channel number

FIG. 2

y-ray spectrum of neutron-rich cerium isotopes measured inposition D 6 of Fig. 1

REFERENCES

1. R.D. MACFARLANE, R.A. GOUGH, N.S. OAKEY and D.F. TORGERSON,

Nucl. Instr. Methods 7_3, 285 (1969)

2. K. WIEN, Y. FARES and R.D. MACFARLANE, Nucl. Instr. Methods

103, 181 (1972)

3. H. DAUTET, S. GUJRATHI, W.J. WIESEHAHN, J.M. D'AURIA and

B.D. PATE, Nucl. Instr. Methods 107, M9 (1973)

Vol. 11.Nu. 11 Fast Chemkal On-line SepualkHi System SISAK 735

4. K.L. KOSANKE, W. MCHARRIS and R.A. WARNER, Nucl. Instr.

Methods Jll5_, 151 (1974)

K.L. KOSANKE, Thesis, Michigan State University, East Lansing

1973 (COO-1779-76)

5. H.G. WILHELM, H. JUNGCLAS, H. WOLLNIK, D.F. SNIDER, R. BRANDT

and K.H. LUST, Nucl. Instr. Methods 115, 419 (1974)

6. P. PUUMALAINEN, J. ÄYSTÖ and K. VALLI, Nucl. Instr. Methods

112, 485 (1973)

7. R.J. SILVA, J. HARRIS, M. NURMIA, K. ESKOLA and A. GHIORSO,

Inorg. Nucl. Chem. Letters £, 871 (1970)

8. D.C. AUMANN and D. WEISMANN, Nucl. Instr. Methods 117, 459

(1974)

9. K.-H. HELLMUTH and K. VALLI, Nucl. Instr. Methods 1_2_5, 99

(1975)

10. P.O. ARONSSON, B.E. JOHANSSON, J. RYDBERG, G. SKARNEMARK,

J. ALSTAD, B. BERGERSEN, E. KVÂLE and M. SKARESTAD, J.

Inorg. Nucl. Chem. _3£» 2397 (1974)

11. P.O. ARONSSON, Thesis, Chalmers University of Technology,

Göteborg (1974)

12. P.O. ARONSSON, G. SKARNEMARK and M. SKARESTAD, J. Inorg.

Nucl. Chem. 36, 1689 (1974)

13. P.O. ARONSSON, G. SKARNEMARK and M. SKARESTAD, Inorg. Nucl.

Chem. Letters 10, 499 (1974)

14. P.O. ARONSSON, G. SKARNEMARK, E. KVALE and M. SKARESTAD,

Inorg. Nuci. Chem. Letters K), 753 (1974)

15. W.J. WIESEHAHN, J.M. D'AURIA, H. DAUTET and B.D. PATE,

Can. J. Phys. 51, 2347 (1973)

16. R.J. SILVA, N. TRAUTMANN, M. ZENDEL and P. DITTNER, to

be published

17. H. REINHARDT and J. RYDBERG, Acta Chem. Scand. £3, 2773

(1969)

DECAY PROPERTIES OF 1/|3La

T. Björnstad and E. Kvâle


G. Skarnemark and P.O. Aronsson

Department of Nuclear Chemistry, Chalmers Universityof Technology, Fack, S-402 20 Göteborg 5» Sweden

N. Kaf f re l l , N. Trautmann and E. Stender

Insti tut für Kernchemie, Johannes Gutenberg-Uni-versi tät , Postfach 3980, D-65OO Mainz, Germany


Abstract: Y"S ingles and y-y coincidence measurements have been performed

on samples of La produced in thermal neutron induced fission of U.

The nuclide was isolated by means of the on-line chemical separation

system SISAK in combination with a gas j e t recoil transportation (GJKT)

system. From the decay of the strongest y - l ines , the h a l f - l i f e of La

has been determined to be 14.23 ± 0.14 m>n* A number of new y-rays are

reported, and decay scheme involving many new levels has been derived.

INTRODUCTION

The neutron-rich nuclide '*La has been surprisingly l i t t l e studied

despite its relatively long h a l f - l i f e of 14 min. The h a l f - l i f e is

sufficiently long to allow the isolation and the purification of the

nuclide from a fission product mixture by traditional techniques as

precipitation, ion exchange or o f f - l ine solvent extraction.

Gest and Edwards I I ] showed already in 1951 that the lanthanum143

precursor of Ce had h a l f - l i f e of about 19 min. However, the f i rs t143

serious attempt to investigate the decay of La was carried out about

ten years later by Fritze et a l . 12]. They published h a l f - l i f e , 0- -

value and a y-ray spectrum measured by a Nal (Tl)-detector. Since then,

very few additional data on La [3.4,5] have been published.

Therefore we decided to perform experiments involving y-rays singles143

and y-y coincidence measurements on the decay of •'La, u t i l i z ing the

fast on-line chemical separation system SISAK 16,7] in combination

with a gas j e t recoil transportation (GJRT) system [8 ] , These experiments

are part of a larger decay study project on the neutron-rich isotopes

of La, Ce and Pr in the transition region between spherTcal and deformed143

nuclear shapes, bat this paper wi l l exclusively deal with La.

EXPERIMENTAL

Irradiations and chemical separations

143 235The La was produced in thermal neutron induced fission of U

11 7

(450 jag) at the Mainz TRIGA reactor ( f l u x ~ 10 n/cm -s) The target

arrangement was identical to the one described in ref . [ 8 ] .o-ac

In thermal neutron induced f ission of J ? U , the proton number nearest

to the most probable charge (Z ) of the mass number 143 is 56 , corresponding

to the element Ba. The cumulative chain y ie ld of the 13.6 s Ba is

5.20 %, while the independent y ie ld of \ a is 0.57 % and the total

chain y ie ld 5.80 %. Accordingly, i t is advantageous to remove the143 143

primarily formed lathanides, and milk the La from Ba. The chemical143separation system for isolation of La is shown in f i g . 1 . The j e t

gas carrying the f ission products is thoroughly mixed with an HN0,-

solution of pH 1.4 in a s ta t i c mixer. After a degassing step where the

j e t gas and most of the noble gas ac t iv i ty (>95 %) are removed, the

aqueous solution is fed into the f i r s t mixer-centrifugal unit CI . Here

i t is contacted with an organic phase, org 1 , consisting of 0.3 M HDEHP

(bis-2-ethy 1 hexylortophosphoric acid) in kerosene, and the lanthanides

are extracted into org 1 , leaving Ba in the aqueous phase, aq 1 . A delay,

Delay 1, of about 15 s in aq 1 allows 50-60 % of the present 3Ba to143 142 142

decay into "Xa while the growth of La from the 10 min Ba is

kept at a reasonable leve l . The grown-in lanthanides (mainly

' La) are extracted into org 2 (in C2) which has the same

chemical composition as org 1 . In a delay, Delay 2 , of 200 s in org 2

more than 96 % o f the 42.1 s La decay to the longlived Ce. In C3,143

the La is backextracted to an oxidizing aqueous solution of 1 MHNO-, 0.1 M H,SO. and 0.05 M K,Cr,0, , leaving Ce (IV) in org 2 . Finally

Jj/j5 C t C C I

the La is adsorbed on a cation exchange column (Dowex 50Wx4, 50-100

mesh), and the y ray counting is carried out directly on this column.

The specifications of the chemicals used are the same as given in

ref. 19].

ry-singles and y~y coincidence measurements

The polypropylene counting cell containing the ion exchanger resin

is identical wfth the flow cell described in ref. [9 ] . The cell allows

detect Ton of low energy T-trans i t ions down to 18-20 keV.

Since the production-separation system used in the present work is142a real on-line system, the build-up of La on the cation exchanger

143w i l l , to an increasing extent, affect the measurements of La. To

reduce this contamination, the resin was frequently renewed during the

Y-Y coincidence measurements.

The ha l f - l i f e determination was carried out by the traditional

technique of sampling (on the ion exchanger) and subsequent decay

measurements. The measuring periods were designed so as to fac i l i ta te

correction for possoble long-lived components in the y-ray peaks.

The Ge(Li) Y~ray detectors used had an energy resolution at 1332 keV

of 2.3 keV and 1.75 keV and relative efficiency of 23.6 % and 6.4 %,

respectively. In addition, to improve the energy resolution in the low

energy region an X-ray detector was used (resolution at 122 keV ~ 600

eV).

The electronic equipment was identical with that described in ref.

[9]-


Y~ray spectra

A typical Y-ray singles spectrum is shown in f i g . 2b. Fig. 2a shows

the low energy part of the spectrum (recorded with the X-ray detector).il,?

The Y-ray peaks belonging to "la and the most intense peaks from the

contaminants ] i f 2La, ^ ' i ! > 5 ' i WCe and 1it6Pr are labeled with energies

(in keV).

Ha l f - l i fe

Two of the most intensey-ray peaks at 620.6 and 621.7 keV were used

for the ha l f - l i f e determination. These peaks form a doublet, but a

computer f i t shows identical half-l ives for the two peaks. In order to

obtain better counting statistics and to avoid the extra uncertainty

introduced in the f i t t ing procedure, the total net area under the

r 4.

doublets was used. Five runs were performed. The half-life was

calculated by a least squares fit to the decay data, and the results

are summarized in table 1,

The final value assigned, 14.23 ± 0.14 min, is the weighted mean of

single values given. It agrees well with the literature values of

14.0 ± 0.1 min [2] determined by energy gated ß-measurements on

chemically purified samples, and 14.32 ± 0.73 min [4] obtained from

computer-fit of decay curves from ß-measurements on mass separated

samples.

Y-ray energies

143,The y-ray energies attributed to La are shown in table 2 together

with relative intensities and coincidences.

143I|B, we have used threeWhen assigning the yray energies to

criteria. The Y-ray should n£,

1) have coincidence(s) with Y-ray(s) unquestionably belonging to

' 4 \ a (mainly those with I > 20 % ) , ' ^ ,:>,

2) show the right half-life (14 ± 2 min), ^ 7 t

3) fit into the level scheme constructed from the (d,p)-reaction data

by Lessard et al. [10], the thermal neutron capture Y'data of

Groshev et al. [11] and the Y"T coincidences from the present work.

Criterion 1 or 2+3 have each been accepted as sufficient. If only one

of the criteria 2 or 3 is fulfilled, the assignments is still in

question.

The energy calibration curve was composed of three parts: 0-300 keV

with the standard nuclide i 6 0Tb, 300-2000 keV with standards 6 0Co, 8 8Y

and Cs, and above 2000 keV where Co was used as a standard. The

final curve is constructed by a linear fit within each of the energy

regions.

In some cases the energies given are less accurate. The peak at

475.5 keV is the weak component in a doublet with the stronger 476.7

keV, and.both the energy and intensity are estimated from the14?coincidence spectra. For the 2056 keV peak the probaple La-component

is completely covered by the stronger La-component, and is found

just as a shorter lived part of the decay curve of 2056 keV. The 1299

keV peak is not clearly shown in the singles spectra, and is found only

rthrouh the coincidences. Generally, the energy values given in the

regions 0-300 keV, 300-2000 keV and above 2048 keV are the mean of 3,

5 and 2 single values respectively.

Decay scheme

The decay scheme derived is shown in fig. i, and for comparison

the level scheme from refs. [10] and [11] is parallelly drawn. The Q_-

value of 3.3 ± 0.1 HeV is the one measured by Fritze et al. [2], It is

in agreement with the value calculated by Viola et al. [12] of 3.383

MeV.

Some general comments on the decay scheme are necessary. The scheme

is drawn with the sybols used in Nuclear Data Sheets. Many of the

coincidence peaks are rather weak, and in many cases it has been

difficult to decide whether the coincidences are random or true cascade

coincidences. In the cases where cross checking have revealed the same

result, the coincidences have been accepted as true.143

The lowest lying level in the ground state multiplet of Ce is

measured by means of the atomic beam method to be 3/2 [13] in consistence

with angular correlation data and decay schemes of this nuclide [I1!].143

Ce has 58 protons and 85 neutrons in the nucleus. According to the

shell-model theory each of the three neutrons outside the closed shell

82 is in the f-iij' state, and the coupling between three identical

particles in the same j-state usually gives I = j - 1 [15]. This should

yield I = 5/2 which is not observed. The measured anomalous spin of 3/2

might be explained if long-range Majorana forces were in operation [13].

In a polarization experiment, Graw et al. [16] have found a low-

lying excited level of spin and parity 7/2 « According to Lessard et al.143[10], who studied the levels in Ce by means of the (d,p)-reaction,

this level has probably an excitation energy of 20 keV and it is assumed

to be identical with the 18.9 keV level derived in the present work.

The transition from this 18.9 keV level to the ground state should thus

be an E2 transition, which is almost completely converted. This explains

the absence of an 18.9 keV line in the measured Y~ray spectra.

The next level found at 40 keV in the (d,p) reaction [10] may

correspond to the state observed by us at 42.3 keV; it is depopulated

by rather strong yray transitions of 42.3 and 23.4 keV to the ground

r rstate and the 7/2~ level at 18.9 keV, respectively. As the (j-1) state

is also expected at a low energy we would propose Jn » 5/2 for the

42.3 keV level. Concerning the other levels, although some of them are

close in energy to those found in the (d,p) reaction, no definite spin

and parity assignments can be deduced.

ACKNOWLEDGEMENTS

The autors express their gratitude to Professors G. Herrmann,

A.C. Pappas and J. Rydberg for their interest in this work. We wish to

thank Mr. R. Heimann for his assistance during the experiments and the

staff of the Mainz TRIGA reactor for the irridations. Finacial support

from the Swedish Atomic Research Concil, the Bundesministerium für

Forschung und Technologie and the Norwegian Research Council for Science

and the Humanities is gratefully acknowledged.

REFERENCES

[I] H. Gest and R.R. Edwards, National nuclear energy series, PlutoniumProject Record, IV, 9 (McGraw-Hill Book Co., Inc., New York, p.11441951

[2] K„ Fritze, T.J. Kennett and W.V. Prestwich, Can. J. Phys., 39,662 (1961) ~~

[3] A. Ohyoshi, T. Tamai and M. Shinagawa, Bull. Chenu Soc. Jap., M»,3*89 (1971)

[k] B. Ehrenberg and S. Ami.il, Phys. Rev., Ç6, 618 (1972)

[5] K. Buchtela, Atomkernenenergie (ATKE), 22, 268 (197*)

[6] P„0. Aronsson, B.E„ Johansson, J. Rydberg, G. Skarnemark, J.Alstad, B. Bergersen, E. Kvâle and M. Skarestad, J. inorg. nucl.Chem., 36, 2397 (197*0

17] P.O. Aronsson, Thesis, Chalmers University of Technology,Göteborg 197*

[8] N. Trautmann, P.O. Aronsson, T. Björnstad, N. Kaffrell, E. Kvâle,M. Skarestad, G. Skarnemark and E. Stender, Inorg. nucl. Chem.Lett., jn, 729 (1975)

[9] G. Skarnemark, P.O. Aronsson, T. Björnstad, E. Kvâle, N. Kaffrell,E. Stender and N. Trautmann, J. inorg. nucl. Chem., in print

[10] L. Lessard, S. Gales and J.L. Foster, Jr., Phys. Rev., C6, 517(1972) ~~

[II] L.V. Groshev, V.N» Dvoretskii, A.M. Demidov and M.S. Al'vash,Sov. J. Nucl. Phys., J£, 392 (1970)

[12] V.E. Viola, Jr., J.A. Swant and J. Graber, Atomic Data andNuclear Data Tables JjJ, 35 (197*0

r[13] I. Maleh, Phys. Rev., J38, B766 (1965)

[\k] P.R. Gregory, L. Schellenberg, Z. Sujkowski and H.U. Johns, Can.J. Phys., <£, 2797 (1968)

[15] M.G. Mayer and J.H.O. Jensen, Elementary Theory of Nuclear ShellStructure (John Wiley & Sons Inc., New York 1955)

[16] G. Graw, G« Glausnitzer, R. Fleischmann and K* Weinhard, Phys„Letters, 28B, 583 (1969)

r 8.

Run no

1

2

34

5

Hal f - l i fe (min)

14.30 ± 0.25

14.14 ± 0.33

13.86 ± 0.34

14.31 ± 0.32

14.43 ± 0.32

Ha l f - l i f e (min)mean value

14.23 ± 0.14

Table 1

Ha l f - l i f e of La as determined from the double peak

of the two most intense y-ray energies 620.6 and 621.7 keV.

J

r 9.

621.7 « 0.3

6*3-9 « 0.3774.9 i 0.3798.3 « 0.3860.3 -. 0.3919.3 « 0.59«.7 « 0.5

1053.2 « 0.3106k.2 ± 0.3

1076.5 * 0.3

1086.8 • 0.3

1122.9 i 0.3

1139.6 • 0.3

11*6.0 > 0.3

11*8.6 t 0.3

1164.9 « 0.3

1167.7 » 0.5

1201.1 * 0.3

12*0.2 * 0.3

1299.0 « 1.01*02.6 « 0.51*23.6 < 0.511.75.5 ».0.3

1556.6 * 0.31592.5 * 0.31611.7 « 0.3

1707.9 « 0.3

17*0.9 * 0.3

1838.1 « 0.)

1876.1 • 0.3

1878.* • 0.3

1938.0 i 0.3

1961.5 • 0.3

19BD.* i 0.3

200*.1 « 0.3

2056 • •

2066.3 i 0.5

2385.2 " 0.5

2500.0 » 0.5

2625.0 «0.5

2710.2 i 0.5

2825.6 « 0.5

808.1 • 0.3°

13*6.« « 0.3c)

16611.2 • 0.3c'

coincidence«' tittrmtttre valves 121C MWI • « )

2}.* • ».3

«.3 « «.3

30».J • f.S

«33.< • 0.5

*S*.2 . 0.5

*75.5 « 1.0

*76.7 ± 0.3

527.5 « 0.3

560.1 i c.3

581.9« 0.3

620.6 i 0.3

1.7*.l

<57.66.<i

8.8

8.2

100

*7.9

71.6

15.1

*7.8

5.19.63.0

25.62.8

15.2

2.613.67.9

31.0

39.91 * . *

* . 2

9.75.3

<22.32.37.9

•2.3*.53.5

16.*

*.98.*

9.68.7

16.0

«0.1

14.2

10.22.72.67.2

29.312.8

3.5

5.1

, 77*.9 I»), !O53.IM, 1122.9b)

15S6.K.1. 19JI.11.)

620.6U). 774.)l>). KS3.2M, I5S*.6(.)

l93t.O(.)

Not gated

Not gated

560.1(»I. 620.61«), 6*3.91*). 1402.61.)

1MI .7M

IO53.2M

560.1(5). 621.7(»), 1611.71«)

621.7<«>. 1139.61«)

300.21.). 45*.2(5). *76.7(*>. *20.6(s).

6*3.9(*»

433.K"). 1053-2(51. 1D76.5I»)

23.41s). «2.31»), «33.11«). *S*.2(5). S60.1U)

1064.21«), 592.5!-). I74D.9M. 2066.31.)

*76.7(s). 527.S(s). 942.7W. 10B6.8(«). 1876.11»)

1878,*(n). 203*.H-l

433.K.I . *5*.2(5). 560 . IM, 1592.SM. 17*0.91.)

23.41.). 42.31-). 300.21«!. 860.3M. 124C.2<-!

300.2M. 860.3M, 12*0.2(0)

798.31«)

0.2H

».**«

«.«25

0.915

. •7

.17

-SB

.70

2.22

2.46

2.56 :

2.85 :

' • - * •

: 0.01

: g.ai

r ».02

: 0.«

, 0.02

I 0.0*

: 0.0*

0.03

0.03

0.03

0.03

0.0«

».08

0.2*

0.57

3.26

0.59

0.35

0.06

0.13

0.27

0.15

Not g*led

23.4(>), *2.3to), 475.SM. 5B1.9(<). 1211.Sf.)

1423.6M. 1475.5W

581.9(<). 1475-5M

Not gated

23.4M. *2.3(.)

1I48.6M

None

I139.6W. 147S.5M

None

None

12991»)

Not gated

not gated

*5*.2(-l, *76.'(«). 6Z0.6(n>, 6*3.9WNot gated

I053.2M. I076.5M. U48.6M

23.*<™). *2.3("), 919.3I»)Not gated

*54.2(r»>, 476.2W. iJ0.6W

Hone

620.61.). U i . J U

None

62I.7M621.7(r»)

Not gated

None

Hone

No[ gated

none

Not gated

None

None

Nune

Hone

None

* ' The error tiaits «re estiwtcd to be * I t * for the strong lines (I > 21 X) m>4 «21 * for the «uk ones.

* - strong, • • nediiw, w • Mcak.

c ' y-wtttrgits showing the right half-life, but not fitted into the decay sen«**.

Table 2

Energies, relative intensities and coincidences observed for y rays

assigned to La.

r r

1*2 • C2H4

noble gases

0 > M HDfHr >n kfiaww

' 025 M HNO3

^ IFP

pH-14

P»I»T 2in**

1 M HNO3 ^ 3 C

0.1 M H2SO« ^ « |O.OS M K2Cr2O7

HjO

Fi gure 1

The chemical separation system used for the isolation of La.

H = stat ic mixer for gas and l iquid, Dg = degassing unit , NG = noble gases,

C1-C3 = mixer-centrifugal separator units, FP = fission products,

C = counting cell (DOWEX 50Wx4, 50-100 mesh), D = detectors

r r2000

1000 -

(0

I

CHANNEL NUMBER

Figure 2a

Low energy y-ray spectrum of La

300

J

r 12.

' " " l a singles spactrvm

Envryr rang« 1000 2000 K«V

O 5 KaV per channel

3000

CHANNEL NUMBEH

1*3,

Figure 2b

Y-ray spectrum of La covering the energy range 30-3000 keV

r 13 rO. • S J «*V \

n "-Si»M'

Figure 3

H 3Decay scheme of La

rRadiochimica AcU 13,96-103 (1976)© by Akademûche Verlag»ge.ell»chaft, Wiesbaden 1976

Decay Properties of Some Neutron-Rich Praseodymium Isotopes

By G. SKAR.NEMABK», E . STESDER", N. T R A U T M A S S » « , P. O. ARONSSOS«, T. BjÖRRSTAD«", » . KAPFKEIL»»,

E. K V À I E * " and M. SKABESTAD***, The SISAK Collaboration

With 6 figures. (Received June 11,1976; in final version August 20,1976)

Summary

Neutron-rich Pr isotopes produced in the thermal neutron-induced fission of œ U have been investigated by means ofy-y coincidence experiment«. The nuclidea have been sepa-rated from the fission product mixture, using the fast chemicalseparation system SISAK in connection with a gas jet recoiltransport system. The results include assignments of severalnew y-ray energies and partial decay schemes for u 'Pr, >4'Pr,'«Pr and "»Pr.

Zusammenfassung

Neutronenreiche Praseodym-Isotope, hergestellt durch Spal-tung von aiU mit thermischen Neutronen, wurden mittelsy-y Koinzidenzmessungen untersucht. Die Abtrennung derAktivitäten aus dem Spaltproduktgemisch gelang durchAnkopplung eines Gas-Jet-Transportsystems an das schnellechemische Trennverfahren SISAK. Mit den erhaltenen Er-gebnissen können einige neue y-Linien zugeordnet und partielleZerfallsachemata für "'Pr, >«Pr, '"Pr und I50Pr aufgestelltwerden.

Résumé

Quelques isotopes de praseodymium riches en neutrons pro-duits dans la fission de S35U avac des neutrons thermiquesont été étudiés par des mesures de coïncidence y-y. Lesactivités ont été séparées de l'ensemble des produits de fissionen utilisant le système SISAK de separation rapide chimiqueen connection avec un système de transport «jet de gaz t.Les résultats permettent d'attribuer plusieurs energies y nou-velles et d'établir des schémas de désintégration partiels desisotopes '"Pr, '«Pr, >«Pr et «oPr.

1. Introduction

From the neutron-rich Pr isotopes, the nuclides M'Prand MSPr with half-lives of 12 min and 2.2 min,respectively, were first identified by HOFFMAN andDANIELS [1] who separated praseodymium radio-chemically from fission products. For the assignmentof the 12 min Pr-activity to mass number 147 theyalso performed (y,p)-veactions on enriched MSNd. Inaddition to the half-lives they obtained a partial decayscheme for "'Pr and {»„-values of 2.7 ± 0.2 MeV and4.5 ± 0.4 MeV for »'Pr and 1MPr, respectively. Thedecay of u 'Pr has also been thoroughly studiedrecently by PINSTON et al. [2] using samples whichwere mass-separated from fission products and byDORTKENS and DORIKENS-VANFBAET [3] who obtainedthe activity via a M*Nd (y,p)-reaction.Half-lives and some y-ray energies of lMPr (2.0 min)and MtPr (2.3 min) were reported by OHYOSHI el al. [4J.

These uuclides were isolated from fission products byan electromigration technique. Shortly after that,ABONSSOK el al. [5] published energies and relativeintensities for several new y-lines attributed to thedecay of "»Pr (2.2 ± 0.1 min) on the basis of y—ycoincidence measurements. The praseodymium sourceswere produced by the milking technique on ceriumfractions chemically separated from fission productmixtures. The presence of u fPr in the milked praseo-dymium fraction could be excluded due to a delaytime between production and separation of the ceriumisotopes, in which IMCe had decayed.Some evidence for an ~ 2.3 min activity attributableto »»Pr was found by HOFFMAN and DANIELS [1] inirradiations of 1S0Nd with bremsstrahlung. The exist-ence of a 2.3 min M>Pr was confirmed by VAN KLINKENand TAFF [6] by means of the IMNd (y,p)-reaction.They proposed a partial decay scheme and deter-mined the R v a l u e as 3.0 ± 0.2 MeV.The nuclide l wPr was first identified by WARD el at. [7]after bombardment of enriched l w Nd with 14 MeVneutrons. They determined the half-life as 6.1 s. Ina study of the decay of 1S0Ce, ABONSSON el al.[S] havealso found a half-life of 6.2 s for the daughter producti6opr from thg growth-and-decay curve of the onlyknown y-ray transition assigned so far to the decayof IMPr, which occurs at 131 keV.

• Department of Nuclear Chemistry, Chalmers University ofTechnology, Fack, S 402 20 Göteborg 5, Sweden.** Institut fur Kernchemie, Johannes Gutenberg Universität,Postfach 3980, D-6500 Mainz, Germany.*** Department of Nuclear Chemistry, University of Oslo,Oslo 3, Norway.

1. D. C. HOFFMAN and \V. R. DANISLS, J. inorg. nucl. Chem.86,1769 (1964).2. J. A. PrasroN, R. RoDssnxE, G. BAUXEUD, J. BLACHOT,J . P . BOCQÜET, E. MONNAND, B . PFEIFFER, H . SCHRADER a n dF. SCHUSSLEB, Nucl. Phys. A 846, 395 (197S).3. M. DORIKENS and L. DORIKENS-VASPRAET, Z. PhysikA 275, 375 (1975).4. E. OHYOSBT, A. OHYOSHI, T. TAKEKI and M. SHTNAGAWA,Nucl. Sei. Techn. 10, 101 (1973).5. P. O. ABOSSSON, G. SKABNEMAKK, E. KVALE andM. SKARESTAD, Inorg. nucl. Chem. Letters 10, 753 (1974).6. J. VAN KUINKEN and L. M. TAFF, Nucl. Phys. A » , 473(1967).7. T. E. WARD, N. A. MORCOS and P. K. KUEODA, Phys. Rev,C 2, 2410 (1970).8. P. O. ARONSSON, G. SKARKEMARK and M. SKARESTAD,Inorg. nucl. Chem. Letters 10, 499 (1974).

J

rDecmy Properties of Some Neutron-Rich Praseodymium Isotope«

"Pr

£ £

wMuiX1200 WOO 16B0

Channel number

SUS

3

un 3IMS1IH> •U M 3

517 O

•M»

31« 7

214 1

127 *

1800 "NU

Fig. 1. y-ray singles spectrum of the neutron-rich Pr isotopes. Fig. 2. Partial decay scheme of >"Pr. The scheme is based onmainly M'Pr, M»Pr and l o Pr the coincidences shown in Table 1

The present investigation has been carried out usingvhe continuous radiochemical separation techniqueSISAK [9] in combination with a gas jet recoil trans-port system [10]. y—y coincidence measurements havebeen performed on "'Pr, "»Pr, "»Pr and "°Pr withchemically separated fractions.

2. Experimental procedures

The radionuclides studied in the present work wereproduced by thermal neutron-induced fission of aiVin the Mainz TRIGA reactor. The fission productswere transported with a gas jet to the SISAK system,and the Pr fraction was separated from the fissionproduct mixture as described in ref. [11].The y-ray singles and coincidence measurements werecarried out by ujing two Ge(Li)-detectors with relativeefficiencies of 23.6% and 6.4°/0 and an energy resolu-tion at 1332 keV of 2.3 and 1.75 keV, respectively.In addition, standard electronic equipment with thepossibility of spectrum stabilization was used. Theresolving time of the coincidence set-up was 2T = 25 nsThe coincidence events • ?re stored in a 4K x 4Kmatrix and subsequertv transferred, event by event,onto magnetic tape. All data were evaluated by meansof appropriate computer programs.

3. Reselts and discussion

From the y—y coincidence data obtained, decay»ehernes have been deduced for the nuclides "'Pr,"»Pr, "»Pr and "°Pr.

A typicaly-ray singles spectrum of the three longer-livednuclides "'Pr, "»Pr and "'Pr is shown in Fig.l. Thedecay of the various nuclides is briefly discussed below.

3.1. The decay of "7Pr

The decay scheme of "'Pr is shown in Fig. 2. Thescheme is based on the coincidence data given inTable 1. In order to discuss the present scheme, wehave to review some of the earlier reported data onthe decay of "7Pr and excited levels in M7Nd.With a "*Nd (d,p)-reaction, WIEDNER et ai. [12] haveobserved nine states in U7Nd below 600 keV. Thefirst excited state located at 78 keV which is in agree-ment with the earlier reported decay scheme ofHOFFMAN et al.[I]. This disagrees with the results ofRoussnxE et al. [13] who have found an energy of50 keV for the first excited state in the "»Nd (n,y)-reaction. However, if we consider the assignment fromthe (d,p)-reaction as an erroneous interpretation of the50 keV level as the ground state, while the real groundstate has not been excited in the one-nucleon transfer

9. P.O.ABONSSOÎ», 6.E.JOHANSSON, J.RYDBEKG, G.SKAKNE-MARK, J. ALSTAD, B. BEROERSEN, E. KVALE and M. SKARE-STAD, J. inorg. nucl. Chem. 86, 2397 (1974).10. N. TBADTM '.NN, P. O. ARONSSON, T. BJÖRNSTAD, N. KAF-FRELL, E. KvitE, M. SEABESTAD, 6. SKAKNEMARK andE. STENDER, Inorg. nucl. Chem. Letters 11, 729 (1975).11. P. O. AKOSSSON, G. SKABNEMARK and M. SKAEBSTAD,J. inorg. nucl. Chem. 86,1689 (1974).12. C. A. WIEDNER, A. HEUSLER, J. SOLP and J. P. Wtnm,Nucl. Phys. A108, 433 (1967).13. R. RoussiLLE, J. A. PTNSTON, H. BÖRNE», H. E. KOCHand D. HECK, Nucl. Phys. A 846,380 (1975).

r 100 G. SKABHDCAKK, E. SIBBD», N. Taurmura. P. O. Atoamow, T. Bionsui». H. K ü m o , B. «Mi«««*» Swi«»i>

Table 1. JVte eoiMÜfeMe Art» renttnv from (he too* of "»Pr. • 1=strong) indicates that th» £ • * « * « * " * * " * • ( * 2 £ ü !deviations) in the background, m ( = medium) indicates 1-3» and w (=weak) leaf than la . The mtawbei McdrjereWtmt

intensities

49.977.986.5

127.9186.7214.5249.0314.7328.8335.7389.1413.7467.0477.9517.0554.7577.9627.5641.4942.2996.0

1083.51136.51183.01264.31300.41311

ül.5sM A86.7M9 O4z.V5.4-7.66.9

100.020.025.110.35.38.9

26.523.033.878.3< \84.45.88.5

_7.95.17.9

—

s

ss

m

W

S

S

s

s

s

sss

ss

s

s s s s • s

s s m

a

s s a ns s•

as *

as sas s

s

s

The uncertainties in the energies are estimated to ± 0.3 keV, and in the intensities to ± 10% for the stronger ^-transitions(Iy > 30%) and ± 20'/, for the weaker ones.The coincidence matrixes should be regarded only as a presentation of the raw, unprocessed data obtained. Because the symbolss, m and w refer only to the statistical significance of a certain peak (which is not always the same if gating on the weak or thestrong component of a cascade composed of two y-rays of different intensity), the matrix is not completely symmetric

Table 2. The coincidence data restating from Ike decay of lnPr. For explanation of the letters in the matrix, see Table 1

Wl.) • O O O O C p ^ a O C Q Q O—M f— —• O 1Q

• • «S n ci ç a «

M ! I" I i148.0171.5247.0301.8450.6615.4636.8660.3697.8721.5825.5869.4903.5947.3

1023.21248.81357.91381.621332635

< 1< 10.7

100.05.35.42.53.6

10.98.33.17.53.43.49.06.6

10.44.7

m m ss

J

rDecay Properties of Some Neutron-Rich Praseodyaiam Isotope* !0S

Tabled The coincidence JaUrenllirj from Oeitayol'-Pr. For explanation of the letter» in «•»•atrix, seeTaMel

- o t t * o . » e e « i o « n « j o t « - go o> « » «

s a s s s s i § g s s s s i i s s s s i l £ i93.1

106.6IIS.7138.4156.0162.4165.0182.6207.6212.5224.2227.3238.6258.2264.8313.1316321332.8365.9366.3406.3432.6

30.686.042.6

100.017.034.193.«10.119.9< 512.414.011.944.4

____53.235.735.733.141.3

m• a

reaction, a systematic shift of 50keV in die levelsequence from the (d,p)-experiments has to be takeninto account. Doing this, the (d.p)-data are in excellentagreement with the (n.y)-data. This is further sup-ported by the results of the present work.The present decay scheme disagrees with the schemaof HOFFMAN et al. [1] with respect to the energy ofthe first excited level and the 0.61-0.335-0.105-

0.127-0.078 MeV cascade which ia not seen by us.Our scheme is in good agreement with the independ-ently determined schemes of PINSTON et al. [2] andDOBXKKNS et al. [3]. There is one disagreement con-cerning the reported 1261—49 keV cascade. Althoughwe have seen the 1261 keV y-line in our singles spec-trum, we have found no evidence for a coincidencerelationship with the 49 keV peak.

. «03 •

• am.»

FiE 3 Partial decay scheme of '«Pr. The scheme is based on Fig.4. Partial decay scheme of »Pr. The scheme is based on8 the coincidences shown in Table 2 " - -»î~M-~- •»»— - T.M. 9

ythe coincidence shown in Table 3

r102 G. SKAMEMABK, E. Smcon, N. TBAonujnr, P. O. ABOHSSON. T. BJÔBXSTAD, S. Karmau* E. Kvluand M- SBAURAD

Table 4. The enindiaux «Vite rauUtKj fnm Ike decaf of «Pr.For explanation of the letters in the matrix, see Table 1

s sott

S

lis•2

£5

Iw

|

g

SW.« ) '

Si ù-,

130.2251.2469545.9553.3720.6722.4804.485? 7931.5

1061.6

100.013.4

-

15.9-

-

-

62.725.518.112.4

s m a w a a aa a wm ms

s

sm

Fig. 5. Partial decay scheme of u*Pr. The scheme is based onthe coincidences shown in Table 4

3.2. The decay of "»Pr

Table 2 shows the coincidence data obtained for lwPr.The corresponding decay scheme is given in Fig. 3.Spins and parities are taken from the 1MNd (t,p)-reac-tion data reported by CHAPMAN el al. [14] and the"»Nd (d,d')-reaction data of VAN DEB BAAN et al. [15].The levels at 301.8,752.4,999.6,1171.2and 1683.4 keVagree fairly well with those of the reaction experimentmentioned above, whereas levels at 917.2 and 2074.6keV may correspond to the 911 and 2086 keV statesfound by CHAPMAN el a!. [14].

The 1020 keV level found in the (d,d')-experiment [15]should be identical with the 1023.2 keV state observedby us. The differences in the level energies betweenthe (d,d')-reaction work and the present decay studyare typ;»ally 0 - 3 keV. Thus our level at 1249.0 keVand the (rl,d')-reaction level of 1240 keV may not bethe same. Therefore this state at 1249.0 keV and thelevels at 1659.9, 2434.9 and 2936.9 keV seem to befed only in the ß--deeay.

3.3. The decay of "»Pr

The present results confirm the earlier reported half-life of 2.3 min [1] of "»Pr.The coincidences obtained for IMPr are shown inTable 3. From these data a partial decay scheme,presented in Fig. 4 has been deduced. This schemeincludes the y-lines at 108.5, 138.4 and 165.0 keValready assigned to "*Pr by VAN KLINKEN and TAFF[6]and several new, weak transitions. The levels are ingood agreement with the results of HECK et al. [16],from the 14>Nd (n,y)-reaction and with a study of thelevels in u*Nd using the (d,t) and (sHe,a)-reactionsby B r a u e t ai. [17].

3.4. The decay of l wPr

In this experiment, 6.2 s IMPr was milked from a Cefraction containing 4.0 s l wCe[8]. This half-life isclose to die Iowvr limit accessible with the presentsystem, and in combination with the low cumulativefission yield of IMCe, only low-intensity samples canbe obtained. Therefore only a preliminary decayscheme can be deduced from the obtained coincidencedata, which are summarized in Table 4. The schemeis shown in Fig. 5.The levtls at 130 and 381 keV correspond to the2+ and 4+ levels of the K = 0 ground state rotationalband [15]. The 676 level is assumed to be identicalwith the 672 keV level observed by V A S DER BAANet ai. [15] in the '»Nd (d,d')-reaction and the 675 keVlevel of CHAPMAN el ai. [14] in the 1MNd (t,p)-reaction,and may be interpreted as the 0+ member of theK = 0 0-vibrational band. The 848 keV level (2+, 1~)of VAN DEB BAAN el al. is presumably identical withthe 851 keV level (2+) of CHAPMAN et at. la the (d,d')-reaction work [15] they found that the cross sectionfor the 848 keV level (which is interpreted as the2+ member of the K = 0 /?-vibrationaI band) washigher compared with the similar level in "^Sm. 'Wieyconcluded that the 848 keV peak may be an unresolveddoublet which also contains some contribution froma 1~ state. In our decay work, this level has split upinto two levels with energies of 851 and 853 keV.An exact spin assignment of these two levels is notpossible. However, from the decay pattern of the twolevels the 853 keV state may be interpreted as the1- member of the K = 0 octupole band while the

14. K. CHAPMAN, E. MCLATCHIE and J. E. HITCHING, Nucl.Phys. A186,603 (1972).15. J. G. VAS DER BAAN, P. R. CHBISTKSSKS, J. RASKDSSENand P. O. ÏJOM. Private communication (1975).16. D. HICK, Private communication (1975).17. D. G. BUBKE, J. c. WADDINOTON, D. E. NELSON andJ. BUCKUY, Can. 3. Phys. 51, 455 (1973). J

ri>«c»y PMpertis ef Scsu Neutron-Rich PnModymiiim Intopw

3 - member at 935 keV should be identical with the3~ level at 931 keV in rcf. [151.Then, the 851 keV state can be tentatively attributedto the 2+ member of the /}-vibrational band.The 1062 keV level should be identical with the bandhead of the y-vibration which has been foond at1057 keV by VAN DER BAAN el <d.[l5].In summary, the present results show some interestingcharacteristics of nuclei in the transitional regionbetween spherical and deformed nuclear shapes.However, more detailed investigations are necessaryfor an interpretation of the obtained level schemes.Especially the properties of the excited 0+ states in"*Nd and 1MNd at 917.2 and 676.1 keV, respectively,are of interest and it seems very important to look inboth nuclei for the second excited 0* states, whichhave been found [18,19] in the Sm isotopes as shapecoexisting states.

103

The authors express their gratitude to Professor»G. HciwifANN, A.C. PAWAS and J. RYDBESO for

their intereit in this work. We wish to thank Mr.R. H S U U K N for his assistance during the expriment«and the staff of the Mains TRIGA reator for theirradiations. We also wish to thank Mrs. E. Joiumfor the preparation of the drawings. Financial supportfrom the Swedish Atomic Research Council, theBundeaministeriuai für Forschung i d Technologieand the Norwegian Research Council for Science andthe Humanities is gratefully acknowledged.

18. P. BnianuM «nd N. M. Hranc, Phj». Rev. Lett«» » ,44 (1970).19. G. WntTH, W. K i m m , K. CmtXâMMTttutxomm,G. Hxanumi nod K. E. San , Z. Phjsik A tit, 301(1975).

\

DECAY PROPERTIES OF ' L a

G. Skarnemark and P.O. Aronsson

Department of Nuclear Chemistry, Chalmers Universityof Technology, Fack, S-402 20 Göteborg 5, Sweden

T. Björnstad and E. Kvâle


N. Kaffrell, E. Stender and N. Trautmann

Institut für Kernchemie, Johannes Gutenberg-Uni-versität, Postfach 3980, O-65OO Mainz, Germany

Abstract: T"ray singles and coincidence measurements have been per-144 4 4144

formed on the neutron-rich nuclides La,235

14gLa and La, produced

235in thermal neutron-induced fission of U. The studies were f.icilita-

ted by the combination of a gas jet recoil transportation (GJRT) system

with the rapid on-line chemical separation system SISAK. Half-lives ofliiii

A2.1 ± 0.7 s, 25.2 ± 2.6 s and 8.5 i 1.0 s were obtained for La,

La and La, respectively. Several new Y~ray energies have been

assioned to each of the nuclides, and partial decay schemes are proposedi'44 i4g

for La and La together with a preliminary partial decay scheme

for ^ a .

INTRODUCTION

Until recently, the neutron-rich transitional nuclei in the light lan-

thanide element region, such as La, Ce and Pr, have been only sparingly

studied, mainly due to the lack of proper fast separation procedures.

However, during the last years a number of new separation techniques

have been invented and existing methods have been further refined. This

experimental progress has made possible a new approach for nuclear spec-

troscopy investigations, e.g. in a classically difficult element separa-

tion area like the rare earths.

This paper will deal exclusively with neutron-rich isotopes of the ligh-

test of the rare earth elements, La, although we have also performed

studies on neutron-rich isotopes of Ce and Pr [1, 2].

r 2.

in

The nuclide La was first identified by Amarel et al.[3]» who isola-

ted the nuclide by on-line mass-separation and determined the half-life

to 41 t 3 s by P-ray counting. In two papers, Ohyoshi et al.[4, 5]144

reported on more comprehensive investigations of La. La was separatedfrom fission products by means of an electromigration technique. The

Y-ray measurements were started about 4 minutes after the end of the

irradiation. From the decay measurements of the two most prominent

Y-peaks »'. 397 and 541 keV they determined the half-life to be 42.4 ± 0.6

s. On the basis of this half-life they also assigned seven other Y~ray144energies to La.

Seyb [6] utilized a fast off-line separation technique for light rare144earths and reported on the half-life and some Y-ray energies for La,

while Wünsch et al.[7] briefly reported on a few Y~ray energies for144

La obtained from mass-separated fission products.

For La and La, less conclusive data are found in the literature.

Grapengiesser et al.[8] detected a component in the mass-chain 145,

decaying with a half-life of about 36 s. The element suggested was La,

in accordance with the results of Wilhelmy [9]. Seyb [6] , has indirectly

measured the half-lives of La and La to be 28 ± 3 s and 8.3 s,

respectively, while Fasching [10] found 24 ± 5 s and 15 ± 10 s, also

from indirect measurements.

In two papers [11, 12] , Aronsson et al. reported on the half-life, Y-ray144

energies and relative Y~ray intensities of La and proposed a prelimi-

nary decay scherte. The same papers also presented the first direct half-

life measurements and Y-ray energy data for La and La, as well as

a simple decay scheme for La. The La isotopes were isolated from

fission products by means of the rapid on-line chemical separation tech-

nique SISAK [13, \ !. In a recent publication [15], we have described the

successful combination of a gas jet recoil transportation (GJRT) tech-

nique with the SISAK separation system. In the present paper, we will

report on Y-Y coincidence measurements on La, La and La perfor-

med with this new arrangement.

J

r 3-

EXPERIMENTAL

Irradiations and chemical separations

The La isotopes were produced as fission products by irradiation of

U (450 ug) with thermal neutrons in the Mainz TRIGA reactor, The' 1 1 - 2 - 1

thermal neutron flux amounted to about 10 n cm s .We have used

the GJRt system described in ref. [15]- Fig. 1 schematically shows the

chemidSW separation system used for the isolation of La isotopes. The

jet gas "carrying the fission products is extensively mixed with a

nitric acid solution of pH 1.4 (~90°C) in a static mixer. The gas-

liquid mixture is then fed into a degassing unit where the jet gas is

swept off together with more than 95% of the noble gas activ'ty. In the

first mixer-centrifugal separation unît, CI, all trivalent lanthanides

(mainly La, Ce and Pr) are extracted into an organic phase consisting

of 0.3 M HDEHP (bis-2-ethylhexylorthophosphoric acid) in kerosene

(Shellsol-T) together with some other fission products like Zr, Nb, Ho

etc. After the phase separation, La (and Pr) is back-extracted (in C2)

into an oxidizing aqueous phase consisting of 1 M HNO,, 0.1 M H_S0, and0.05 K-Cr.O-. In this step, cerium as Ce(IV) and the other elements

remain in the organic phase. The Y~ray measurements are then carried out

on the aqueous phase leaving C2. The organic phase runs in a closed

circuit. This requires a cleaning step (C3) in order to prevent accumu-

lation of longer-lived species.

Chemicals. Most of the chemicals were of p.a. grade. The HDEHP was supp-

lied by Farbenfabriken Bayer AG, Leverkusen, Germany, and used without

purification. Shellsol-T (kerosene) was used as organic diluent.

Singles y-ray and yy coincidence measurements

The counting flow cell was a cylinder of polypropylene (diameter 3.5 cm,

thickness 1.0 cm) with diametrically opposite inlet and outlet. The walls

facing the detectors were 0.1 cm thick in order to allow the detection

of low energy transitions (£20 keV).

The T-ray measurements (singles and coincidence) were carried out by

using two Ge(Li)-detectors with relative efficiencies of 23.6% and 6.4%,

and tnergy resolutions at 1332 keV of 2.3 keV and 1.75 keV, respectively.

In addition, standard electronics with the possibility of spectrum

rstabilization was used. The resolving time of the coincidence setup

was 2 T = 20 ns. The coincidence events were stored in a kK x 4K matrix

and subsequently transferred, event by event, onto magnetic tape. The

spectrum analysis was carried out by appropriate computer programs.

Half-life measurements. The half-life measurements of the three nuclides

have been carried out by the traditional technique of sampling and

subsequent decay measurements. For La, the measurement time sequence

was repeated kO times, while the sequence for La and La was re-

peated 150 times.

The half-lives were calculated from the decay of the most prominent

Y~ray peaks with a least squares fit to the decay data, and the value

finally assigned is the weighted mean of the values obtained for the

individual peaks.


A typical y-ray spectrum of the La fract'on is shown in Fig. 2. The

y-ray peaks belonging to La isotopes are indicated together with some

grown-in Ce and Pr peaks. The results obtained for the different nucli-

des studied will be discussed below, nuclide by nuclide.

La

Ha 1 f -1 i f e • The calculated mean value of 42.1 ± 0.7 s from the decay of

the Y-ray peaks at 397-5, 541.3, 585.1 and 844.9 keV is in good agreement

both with the p-ray measurement data of Amarel et al. [3], 4l ± 3 s, and

the value obtained by Ohyoshi et al. [5], 42.4 ± 0.6 s.

144Decay scheme. The Y-rays assigned to the decay of La are listed in

Table 1 together with the observed coincidences. The decay scheme derived

from these coincidences is shown in Fig. 3.

From three and four parameter coincidence measurements (including yrays,

X-rays and fragment .-nasses) on products from the spontaneous fission of1 *>?

Cf, Cheifetz et al. [16] determined the energy of the ground state+ + 144

2 -»0 transition in Ce to be 397.5 keV. The present energy data

agree well with this result as well as with the findings of Ohyoshi et

al. [5], Seyb [6] and Wünsch et al. [7], except for the 260.9 keV peak

J

F[7] which is not seen in the present work.

The present decay scheme is an extension of the scheme previously pro-

posed by us [12]. In that paper we compared the experimentally obtained

levels with the predictions of the variable-moment-of-inertia (VM!)

model [17], concluding that the 938 keV level and the 1523 keV level

probably were the k and 6 members of the ground state rotational band.

However, in the present work we have found a 1523 keV transition. If

this transition de-excites the 1523 keV level, it excludes the spin

assignment 6 to this level. There is, however, a weak coincidence bet-

ween this T"raY an£l the 397 and ikk keV y-rays, thus implying that it

de-excites a 2766 keV ievel instead of the 1523 keV level. This is also

in agreement with the results of Monnand and Fogelberg [19]- Thus the

1523 keV level might still be the 6 level. In order to make definite

conclusions about the spin of this level, y-y angular correlation measu-

rements are a subject of future interest.

Since the Q -value has been measured to be k.1* MeV [18], ß-feeding of

higherlying levels than the 2641 keV level is energetically possible.

Inspection of the y-spectra indicates peak energies even above 3000 keV,

but with too low intensities to be uniquely assigned. Besides the more

conclusive energy data in Table 1, Table k indicates several yray ener-

gies which may be attributed to the ß -decay of La.

La is a difficult nuclide to characterize because of weak

Y-ray transitions. Two of the more prominent peaks at 70.2 and 170.2 keV

have been used for the half-life determination. The decay curves are

shown in Fig. k. It should be mentioned that these "y-rays have been att-

ributed to La only on the basis of their half-life (which is in good

agreement with the half-life obtained from the growth-and-decay curves1 5of the Ce peaks in the La fraction), and that one cannot completely

exclude the possibility that they belong to another lanthanide nuclide

with the same half-life. The final half-life assigned, 25-3 ± 2.6 s is

in agreement both with the values of Seyb [6] (28 ± 3 s) and Fasching

[11] (2k ± 5 s).

Decay scheme. Y~ray energies (with relative intensities) and coinciden-

r 6.

ces are listed in Table 2. From these data we have derived a decay

scheme, which should be regarded as a preliminary version (Fig. 5 ) . It141» 145

seems as if the Z decay of La mainly feeds the ground state of Ce,and the weak y-ray transitions make some of the assignments uncertain.

145Besides the y-ray energies assigned to La in Table 2, Table 4 givesa number of candidates for the same decay proposed on the basis of the

measured half-lives.

146La

145Ha If-life. The two most prominent y-rays in the La ß-decay, 258.5

and 410.0 keV, have been used for the half-life determination. The

decay curves are shown in Fig. 6. The half-life value finally assigned,

8.5 ± 1.0 s, is somewhat lower than the value of 11 i 1 s earlier found

by means of the two-detector-delay (TDO) method [12]. It is consistent

with the results of Seyb [6] (8.3 s) and Fasching [11] (15 ± 10 s ) .

146It should also be mentioned that some of the y-rays assigned to La

show decay curves which indicate the prescence of a weak, longer-lived

component. These y-rays, e.g. the transitions at 183.2 and 503.1 keV,

are, however, relatively weak, and thus the counting statistics are

comparatively poor.

Decay scheme. The y-ray energies and coincidences obtained are listed

in Table 3, and the decay scheme derived from these data is shown in

Fig. 7- Cheifetz et al. [16] assigned energies of 502.3, 410.1 and 258.6

keV to the transitions belonging to the 6 -»4 -* 2 -»0 cascade in

Ce. It is reasonable to believe that these energies correspond to

three of the most prominent transitions found in the present work at

503-2, 410.0 and 258.5 keV. We adopt the spin assignments 2 + and 4 + for

the levels at 258.5 and 668.5 keV, respectively, but we consider the

assignment 6 to the 1171.7 keV level as less conclusive, due to the

relatively strong 3"feeding of this state as well as of the 2 + and 4 +

states. Thus, if the 1171-7 keV level is a 6 state, it can be fed only

from the decay of an isomeric state in La. As previously mentioned,

the decay of some of the y-rays of La indicate the prescence of a

longer-lived component. However, this evidence is too weak to allow any

definite suggestion of such an isomeric state.

r 7.

ACKNOWLEDGEMENTS

The authors are indebted to Professors G. Herrmann, A.C. Pappas and

J. Rydberg for their interest in our work. We are also indebted to

Hr. R. Heimann for kind assistance during the experiments and to the

staff of the Mainz TRIGA reactor. Mrs. E. Jomar prepared the drawings

and Mrs. M. Carlson typed the manuscript We also gratefully acknowledge

the financial support from the Swedish Atomic Research Council, the

Bundesministerium für Forschung und Technologie and the Norwegian

Research Council for Science and the Humanities.

REFERENCES

[1] T. Björnstad, E. Kvâle, G. Skarnemark and P.O. Aronsson,

to be published in J. inorg. nucl. Chem.

[2] G. Skarnemark, E. Stender, N. Trautmann, P.O. Aronsjon,

T. Björnstad, N. Kaffrell, E. Kvâle and M. Skarestad,

accepted for publication in Radiochim. Acta.

[3] I". Amarel, R. Bernas, R. Foucher, J. Jastrzebski, A. Johnsson,

and J. Tiellac, Phys. Lett. 2k^, kQ2 (1967)

ft] A. Ohyoshi, E. Ohyoshi, T. Tamai and M. Shinagawa, J. inorg.

nucl. Chem. ^4, 3293 (1972)

[5] A. Ohyoshi, E. Ohyoshi, T. Ti^ai, T. Takemi and M. Shinagawa,

J. nucl. Sei. Techn. 3, 658 (1972)

[6] K.E. Seyb, Jahresbericht 1973 (Institut für Kernchemie der

Universität Mainz)

[7] K. Wünsch, H. Günther, G. Siegert and H. Wollnik, J. Phys. A:

Math., Nucl. Gen. jj, L93 (1973)

[8] B. Grapengiesser, E. Lund and G. Rudstam, in Proc. Int. Conf.

on the Properties of Nuclei far from the Region of Beta-Stabi-

lity, Leysin 1970, CERN-Report 70-30, p. 1093 (1970)

[9] J.B. Wilhelmy, University of California Radiation Laboratory

Report No. UCRL-18978 (1969)

J

r 8.

[10] J.L. Fasching, Thesis, Massachusetts Institute of Technology,

Cambridge, Massachusetts .1970)

[11] P.O. Aronsson, G. Skarnemark and M. Skarestad, J. inorg. nucl.

Chem. 36, 1od9 (197*0

[12] P.O. Arjnsson, G. Skarnemark, E. Kvâle and M. Skarestad,

lnorS. nucl. Chem. Lett. _l£, 753 (1971»)

[13] P.O. Aronsson, B.E. Johansson, J. Rydberg, G. Skarnemark,

J. Alstad, B. Bergersen.E. Kvâle and M. Skarestad, J. inorg.

nucl. Chem. 36, 2397 ( W O

[14] P.O. Aronsson, Thesis, Chalmers University of Technology,

Göteborg ( 197*0

[153 N. Trautmann, P.O. Aronsson, T. Björns tad, N. Kaffrell,

E. Kvâle, M. Skarestad, G. Skarnemark and E. Stender, Inorg.

nucl. Chem. Lett. }±, 729 (1975)

[16] E. Cheifetz, J.B. Wilhelmy, R.C. Jared and S.G. Thompson,

Phys. Rev. Ç4, 1913 (1971)

[17] M.A.J. Mariscottï, G. Scharff-Goldhaber and B. Buck, Phys.

Rev. J7§, 1864 (1969)

[18] M. Devi Hers, M. Fiche and J. Blachot, private communication,

March 1976

[19] E. Monnand and B. Fogelberg, in Proc. 3rd Int. Conf. on Nuclei

Far from Stability, Cargèse 1976, CERN-Report 76-13, p. 503

(1976)

[20] P.A. Seeger, in Proc. Int. Conf. on the Properties of Nuclei

far from the Region of Beta-Stability, Leysin 1970, CERN-

Report 70-30, p. 217 (1970)

Energy

•TCMObserved coincidences

367.3

397.5

432.1

541.3

585.1

597.9

705.0

735.4

752.1

844.9

952.7

969.0

1102.4

1276.7

1432.3

1489.5

1674.2

1820.3

19433

2009.0

2325.6

2867.1

3100

8

42

12

3

5

11

3

28

8

8

II

3

9

6

3

3

e3

2

2

3

None

432.1(s), 541.3(s). 585.1(s), 705.0(s), 735.4(s),

844.9(s), 952.7(s), 969.0(s), 1276.7(s), 129*.4(s),

1432.3 Is)

397.5(s), 844.9(s), 969.0(s)

397.5(s), 585.Us), 735.4(s), 952.7(s), 969.01s)

397.5(s). 541-3(s), 952.7(s)

Hone

397.5(s)

397.5(s), 541.3(s), 969.0<s)

Not gated

397.5(5), 432.l(s)

397.5(s), 541.3(s), 58S.l(m)

397.5<s). 432. l (s) , 541.3(s), 735.4(s), 844.9<nO

Hone

Hone

397.5(s)

397.5(s)

None

Not gated

None

Not gated

Not gated

Not gated

Not gated

a)The uncertainty in the r-ray intensities is estimated to be ±10% for the strong peaks

fabove 10% relative intensity) and ±20% for the weak ones.

The uncertainty in the Y~ray energies is estimated to be ±0.5 keV below 2000 keV and

±1 keV above 2000 keV. s(=strong) Indicates that the peak is higher than 30 (standard

deviations) of the background, m (^medium) indicates 1~3o and w (=weak) less than lo.

Table 1

Energies, relative intensities and coincidences observed for

Y~rays assigned to La.

5—iÄl fl

j

r 10.

Energy£ y (keV)

48.2

70.2

H / 1

118.4

1*5-3

170.2

189.0

215-3

254.0

403.7

799-8

840.9

890.2

918.2

932.3

959.2

1053.2

1167.8

M. int.*'

40

67. ' ,

100

10

39

5414

7

20

35

<5

4954

11

<516

<5

Observed coincidencesw

Not gated

165.3(sl. 7J9.8I-). 840.9M. 890.2(s). 932-3(s)

tot* »|atwj

7I5-3W. 403-/U). /99 8(s). 840.9UI

70.2(s), 2I5-3M. 403-7U)

254.0M

Hone

IBS.ÖM, 254.0M

None

70.2(s). 117.Kw). H8.4(s). 165-3

70.2M, 116.4(5)

118.4(»)

70.2(s)

None

70.2(s), Il8.4(i). 165.3(»)

Not gated

70.2(«i)

Hot gated

The uncertainty In the T-ray intensities is estimated to be *20î.

The uncertainty ïn the Tray energies is estimated to be 4).5 keV.

For comments, see Table 1.

Table 2

Energies, relative intensities and coincidences observed for145y-rays assigned to La.

EnergyEy (keV)

183.3

258.5

292.4

380.1

410.0

447.2

503.2

515.0

666.4

702.4

785.41043.4

1141.9

2359.8

Rel. in t .. T « >

8

100

1

5

6321

21

22

8

15

4

3

9

13


258.5(5), 292.4M. 410.0(s)

183-3(s). 292.Mn), 380.MS). 410.0(s). 503-2U)

515.0(s), 666.1.(5). 702.4(s). 785-«.(s). 1141-9(s)

Not gated

258.5(s). *10.0{s), 503-2(s)

183.3(s), 258.5(s). 292.4(in), 503.2's), 515-O(s)

None

258.S(s), 380.Ms) . 410.0(5)

258.5(s), 410.Ois)

1 8 3 . 3 M . 258.5(5), 702.4(m)

258.5(s). 666.4(»i)

258.5(s)

None

Hone

Not gated

For comments, see Table 1,

Table 3

Energies, relative intensities and coincidences observed for

Y-rays assigned to La. J

ri l .

Energy

484.

495.523.645.671.

685.

743.76*.

853-880.1216.

1238.

1922.

2156.

2207-

2352.

(keV)

.4

.3

.1

1

.8

86

61

2

78

92

34

Half-life (s) a)

25 ± 4

>60

54 ± 24

25 ± 5

40 ± 10

26 ± 9

38 ± 5

24 ± 7

13 ± 5

26 ± 7

19 ± 6

37 ± 14

24 ± 6

26 ± 7

46 ± 15

27 ± 11

a)The half-lives listed have, in most cases, been determined

by "conventional" decay measurements (i.e. the TDD technique

has not been employed). The uncertainty in the Y~ray energies

is estimated to be ±0.5 keV below 2000 keV and ±1 keV above

2000 keV. It should be noticed that all these lines are very

weak (I , ,\<5% in comparison with the 397-5 keV line from

La) and that some of them may belong to heavier lanthanides

(Nd and Pm).

Table

Unassigned Y"raYs observed in the La fraction.

J

r 12.

C2K4, KNobie gases

Gas jetfrom target

0.3 M HDEHP IN KEROSENE

pH1.4

H2O 0.25 M HNO3 1 M HNO30.1 M H2SO40.05M K2Cr207

1 M HNO30.05 M NH2SO3H0.05 M H2O2

Figure 1

Flow sheet showing the chemical system used for the isolation

of La isotopes. M = mixer, Dg = degassing unit, C1-C3 = mixer-

centrifugal separator units, I = detector cell, D = detectors

J

r 13.

IUZZ

XOfieuio.

zOO

200 400 600 800

1200 MOO 1600

CHANNEL NUMBER

leoo

Figure 2

Y~ray spectrum of neutron-rich La isotopes. The spectrum was

recorded on-line during 30 minutes.

rCounts

1000

100

o: 70.2 keV,Ty2 =26.1 ±3 .5s

A : 170.2 keV, T-,/2 - 24.3 ± 3.9s

T1/2(mean) = 25.3 ± 2.6s

0 10 20 30 40 50Time (s)

Figure 3

Decay curves of some La peaks.

j__^^^_£j

F 15.

Counts

1000

100

o = 258.5 keV,Tv2" 8 0 + 0.5 s* - 410.0 JceV. T-i/2- 9.0 i 0.5 s

T.,» (mean) = 8.5 ± 1.0s

10 20 30 40 50 60

Figure h

Decay curves of the strongest peaks of La.

r 16.

T 1/2-* 2 «a • 4.36 MeV \

[181 \

o5"aj"<o J.5 5 5 . S w S i N ^ 3 ^ .

2643.2

2476.6

1829 5169191674.2

1523.9

1242.4

1102.4 (2*)

938.8 4 +

397.5 2*

0.0 0*144Ce

. Figure 5

Partial decay scheme of La. The scheme is based on the

coincidences listed in Table 1.

145La iß"T1/2=2Ss-»

in,-« ieo)îo?io

CO

1

111

1

1111

1

11

11

11

F 1

40

3.7

(2

0)

21

5.3

(1

4)

25

4. 0

( 7

)18

9. 0

( 5

4)

16

5.3

(1

0)

117.

1 (<

5)

17

0.2

(39

)1

18

.4(1

00

)4i

j 2

(4

0)

70

.2(6

7)

1

1

i

* -

1167.81123-4

959.2918-2

145

639.2

424 2

235.5

170-2

118.470 2

00Co

Figure 6

Partial decay scheme of La. The scheme is based on the

coïncidences listed in Table 2.

r 18.

Q-5.2MIV[20] \

» • •-

1141 380

51

5.0

(22

)

50

3.2

(21

)

18

3.3

(8)

10

43

.4(3

)

78

5.4

(4)

70

2.4

(15

)

29

2.4

(1)

41

0.0

(63

)

1

258. 5

(100)

1810.4

1551.8

1183.51171.71144.2

1043-4 ( i \ 2 * )960.9 ( 3 )

• 5

258 5 2*

00 0*

146,Ce

Figure 7

Partial decay scheme of 1/t6La. The scheme is based on the

coincidences listed in Table 3-

rDECAY PROPERTIES OF SOME NEUTRON-RICH CERIUM ISOTOPES

T. BJÖRNSTAD and E. KVÂLE

Department of Nuclear Chemistry, University of Oslo

Oslo 3, Norway

and

Go SKARNEMARK and P.O. ARONSSON

Department of Nuclear Chemistry, Chalmers University

of Technology, Fack, S-*»02 20 Göteborg 5, Sweden

and


Abstract, y-y coincidence measurements have been performed on neutron-

rich Ce isotopes using the fast radiochemical separation system SISAK

in combination with a gas jet recoil transportation system. The results

include assignments of new y-rays and proposal of partial decay schemes

for 3 min Ce, 56 s Ce and 48 s Ce. The existing decay scheme of

min Ce has been verified, except for a few transitions«

INTRODUCTION

The light neutron-rich lanthanides such as La, Ce and Pr are situ-

ated in the shape transition region between spherical nuclei around

N = 82 and nuclei with substantial deformation beginning around N = 90.

Knowledge about the decay properties of these nuclei is therefore of

considerable interest for the theory of nuclear structure.

So far, however, only a few authors have published nuclear data

on nuclides in this region, mainly due to difficulties in achieving

a proper fast separation procedure.

ja^ÊÊmmmmÊmm

2.

Already in 1943, Hahn and Strassmann [1] isolated a Ce-nuclide

decaying with half-life of about 15 min by precipitation from fission

products.. The nuclide was identified as Ce. Later on, Markowitz145et al [2] isolated 3 min Ce from fission product mixture by employing

a liquid extraction prodecure*

However, no thorough research was performed in this region until

Hoffman et al [3 - 6] made their investigations about ten years ago.

This group isolated Ce from fission products by an extraction procedure

and performed careful studies of Ce, including y-Y and ß-y coinci-

dence measurements. The result was a detailed decay scheme- They also145constructed a partial decay scheme for Ce based on y-Y coincidence

147data. From milking experiments they obtained the half-lives of Ce and

Ce; they were, however, not able to measure any y-rays belonging to

these nuclides.

By means of an electronmigration technique, Ohyoshi et al [7] per-

formed a re-investigation of Ce, and to some extent also Ce. They145proposed a new decay scheme for Ce including one more level than the

scheme given by Hoffman et al„ These results were, however, not based

on coincidence data but only the sum of y-ray energies.

All the above mentioned experiments have been of the discontinuous

type. To improve the experiments one could either employ a fast and fully

automatized off-line technique or make use of a fast on-line chemical sepa-

ration system. The first way was chosen by Seyb [8,9] who utilized an

off-line separation technique where the measurements could start ~ 8 s

after the end of the irradiation* His investigations included half-life

determinations and assignments of some y-ray energies to Ce and Ce.

The second way was selected by Aronsson et al, who developed the

fast, on-line chemical separation technique SISAK [10,11] , which is ba-

sed on liquid-liquid extraction separations.

To contribute necessary experimental data to this rather unexplored

rare earth region, we started a series of experiments on heavy La, Ce and

Pr isotopes produced in neutron-induced fission of U [12-14] employing

the SISAK separation technique. For Ce, these experiments led to half-

life determinations and the first assignments of y-rays to the nucli-

des yCe and Ce, as well as a half-life determination of the pre-

viously unreported nuclide Ce [133 •

A short time ago, we were able to connect the SISAK system to a

gas jet recoil transportation system [15] , thus making reactor irradi-

tions possible. With this arrangement, we obtained much stronger samples

than with the previously used target system [12,14]. This facilitated

the Y-Y coincidence measurements on Ce, Ce, Ce and Ce on

which we report in this paper.

EXPERIMENTAL

Irradiations and chemical separations. The irradiations were performed in' \\ -2-1

the Mainz TRIGA reactor. The neutron flux amounted to about 10 n cm s235

and the target used (450 pg U) was connected to a gas jet recoil trans-

portation system described more in detail in ref„ [15] . The same target

has been used for more than 100 h without any measurable loss in efficien-

cy.

The chemical separation system used for the isolation of Ce iso-

topes is shown schematically in Fig. 1. The jet gas (1:1.4 mixture of

C,Hi and N„) carrying the fission products is thoroughly mixed with a

flow of 1 M HNO-. After a degassing step removing the jet gas and the

noble gases, the liquid is contacted (in Cl) with an organic phase con-

sisting of 2 M HDEHP (bis-2-ethylhexylorthophosphoric acid) in kerosene

(Shell sol -T). In this step, species like Zr, Y and part of the Nb are

extracted into the organic phase, while the light lanthanides remain

in the aqueous phase. After this step, Ce(lll) is oxidized to Ce(IV)

by making the solution 1 M in HNO , 0.1 M in HjSO^ and 0.05 M in KjC^O .

From this solution, Ce (and part of the remaining Nb) is extracted (in C2)

with 0.3 M HDEHP in kerosene. In C3, Ce is reduced and stripped by 1 M

HN03> 0.05 M H 20 2 and 0.05 M NH2SO H; then H^O^ and K2Cr2°7 art «.dded

in the proper quantities to reoxidize Ce, which is then adsorbed on

a HDEHP/PVC column serving as a source for the Y"Y coincidence measure-

ments. Nb remains in the organic phase and does not interfere. On this

column, no other activities than Ce and a small amount of grown-in Pr

could be observed.

Chemicals. Most of the chemicals used were of p.a. grade and manu-

factured by E. Merck, Darmstadt, Germany. The HDEHP was supplied by

1».

Farbenfabriken Bayer AG, Leverkusen, Germany and used without puri-

fication. SheJlsoI-T was used as organic diluent. The PVC beads were

supplied by Kema Nord AB, Sundsval1, Sweden, tonexchanged water was

used throughout.

Measuring equipment and data evaluation. The measuring equipment consisted

of one 16 K and two A K Intertechnique analyzers together with standard

coincidence electronics with the possibility of spectrum stabilization.

The resolving time of the coincidence system was 25 ns» Two Ge(Li)-

detectors were used, one with a relative efficiency of 23% and an energy

resolution at 1332 keV of 2.3 keV, and the other with an energy reso-

lution of 1.75 keV at 1332 keV and 0.72 keV at 122 keV, and a high effi-

ciency in the low energy region. To detect low energy transitions

( £ 20 keV) we used flat, thin-walled counting cells of polypropylene.

All the coincidence data were transferred event by event to magnetic

tape and evaluated by means of appropriate computer programs.


As an example of the chemical purity of the Ce fraction, a Ce

Y"ray singles spectrum is shown in Fig. 2.

The data obtained have facilitated a detailed study of the decay

properties of the nuclides Ce, Ce, 7Ce and Ce. These pro-

perties are briefly discussed below, nuclide by nuclide*

Ce; The coincidences obtained for the 3 min Ce are shown in

Table 1. From these data, a partial decay scheme has been deduced,

as shown in Fig. 3. In this scheme, the levels at 62.7, 787.1, 859.6

and 1210.7 keV are in agreement with the results of Ohyoshi et al [7]

and Hoffman et al [k, 5], No evidence for the level at 300 keV pro-

posed by Ohyoshi et al has been found. The level at ~ 350 keV,

proposed both by Ohyoshi and Hoffman has been split up into two levels

at 3^7.4 and 351.0 keV, respectively. We also propose at new level at

555.0 keV, decaying to the 3^7^ keV level via a rather strong 207.7

keV if-ray and to the 62.7 keV level and the ground state via a

492.2 keV and a 555.0 keV Y-ray respectively.

J

F 5.

1/|6Ce. Although 14 min Ce was carefully studied by Hoffman et al [61,

we have performed a re-investigation of this nuciide. The results

obtained are shown in Table 2 and the decay scheme in Fig. 4. Our scheme

is a verification of the scheme derived by Hoffman et al except for

a few weak transitions namely the Y-rays at 360, 468 and 491 keV pro-

posed by Hoffman et al. We have also found evidence for a 35.0 keV

T-ray deexciting the 35.0 keV level proposed by Hoffman et al [6]. This

y-ray was not observed by the latter group, probably due to a combina-

tion of the low intensity and its situation in the close vicinity of the

light lanthanide Y-ray energies.

Ce and Ce. It is not possible to assign Y-ray energies specifi-

cally to Ce or Ce solely on the basis of their half-lives, because

these are very similar (56 s and 48 s, respectively). In an earlier

work [14] we have therefore reported on coincidence measurements which

facilitated such assignments. In the evaluation of these experiments,

all peaks showing coincidences with the strony 269 keV peak, or with147

peaks in coincidence with the 269 keV peak, were attributed to Ce.

Analoguely, the 292 keV Y-ray peak was used for the assignment of

Y-rays to Ce. The assignment of these two strong y-rays to Ce

and Ce, respectively, was based on their half-lives.

In the present investigation, the detectors used had a better

resolution than in the experiments mentioned abo>.-e. This revealed

that the 269 keV peak is not a single peak, but a closely spaced doublet.

The energy difference between the two peaks is about 0.7 keV.

The doublet nature of the 269 keV Y-ray peak makes the mass

assignment of the two components difficult. However, we have solved the

problem in the following way: The 292 keV Y-ray peak is still attri-148

buted to Ce due to its short half-life (45 + 5 s). The 292 keV Y-ray

is in coincidence with a 130 keV Y-ray, which in turn is coincident

with the 269 keV component having the higher energy. This makes the

assignment of this Y-ray to Ce probable but not unambigous.

Y-rays in coincidence with the 269.7 keV Y-line have then been

assigned to Ce, while those coincident with the 269.0 keV Y-ray147

have been assigned to Ce.

J

r 6.

The assignments are in good agreement with recent findings of

Blachot et al [16] , who used a mass separator to obtain their samples.

The coincidences obtained for Ce and Ce are presented in

Tables 3 and 4, respectively. The corresponding decay schemes are shown

in Fig. 5 and 6.

ACKNOWLEDGEMENTS

The authors are indebted to Professors G„ Herrmann, A.C. Pappas and

J. Rydberg for their interest in our work. We are also indebted to

Or. N. Kaffrell, Dr. N. Trautmann, Dipl. Chem. E. Stender and Mr. R Hei-

mann for kind assistance during the experiments and to the staff of the

Mainz TRIGA reactor. We also gratefully acknowledge the financial support

from the Swedish Atomic Research Council, the Bundesministerium für

Forschung und Technologie and the Norwegian Research Council for Science

and the Humanities.

REFERENCES

!. 0. Hahn and F. Strassmann, Naturwiss. 3J_ (19*»3)

2. S.S. Markowitz, W. Bernstein and S. Katcoff, Phys. Rev. 93 O951»)178 ~

3. D.C. Hoffman and W.Ro Daniels, J. inorg. nucl. Chem. 26 (196'*)1769 ~~

4. D.C. Hoffman and O.B. Michelsen, Kjeller Report KR-76 (1965)

5. D.C. Hoffman, O.B. Michelsen and W.R. Daniels, Ark. Fys. 36 (1966)211

6. D„C„ Hoffman, F.O., Lawrence and W.RO Daniels, Phys. Rev. 172(1968) 1239

7. A. Ohyoshi, E. Ohyoshi, T. Tamai and M. Shinagawa, J. inorg. nucl.Chem. 3ft (1972) 3293

8o K.E. Seyb, Jahresbericht 1972 (Institut für Kernchemie derUniversität Mainz), BMFT-FB K 73-22 (1973)

9. K.E. Seyb, Jahresbericht 1973, Institut für Kernchemie derUnivärsität Mainz, Mainz

10. P.O. Aronsson, B.E. Johansson, J. Rydberg, G. Skarnemark, J. Alstad,B. Bergersen, E. Kvâle and M. Skarestad, J. inorg. nucl. Chem. 36(197*0 2397 ~~

J

rr r11. P.O. Aronsson, Thesis, Chalmers University of Technology,

Göteborg 197*»

12. P.O. Aronsson, G. Skarnemark and M. Skarestad, J. inorg. nucl.Chem. 36 (1971») 1689

13. P.O. Aronsson, G. Skarnemark and M. Skarestad, Inorg. nucl. chem.Lett. J£ (1371*) **99

14. P.O. Aronsson, G. Skarnemark, E. KvSle and M. Skarestad, Inorg.nucl. chem. Lett. 1£ (1971») 753

15. N. Trautmann, P.O. Aronsson, T. Björnstad, N. Kaffrell, E. Kvâle,M. Skarestad, G. Skarnemark and E. Stender, Inorg. nucl. chem.Lett. _H_ (1975) 729

16. C. Devil Hers, C. Fiche and J. Blachot, Centre d'Etudes Nucléaire,Grenoble. Private communication, March 1976

r 8.

Energy

62.7

207232284

347

351

351423.

436.

439.

492.

512.

555.

655.724.

859.1147.1210.

.7

.0

.6

.2

.0

.1

.6

,1

.8,2

.3,0

,9

.3

5

96

fiel, int.*'

lTtt>Observed coincidences

14

1

511

2

54.

12,

2.

12.

4.

2.

3.2.

100.

4.12.

1.

.1

.6

.2

.8

.8

.6

.6

.7

.4

.30

,30

50

2

3

9

207.7(s)

439.8(s)

232.0(5)

62.7U),

62.7(s),

439.8(s)

207.7<s)

436.l(s)

62.7U),6J.7U),

347.2(s)

62.7(s).

62.7(s),

284.6(s),232.0(5)

207.7(m),

423.6U)

351.1(s)

62.5(s)

None

, 232.0(5)

. 724.3U)

, 284.6(5)

207.7(s),

2O7.7(s),

, 512.3(s)

, 439.8(s)

284.6(s),

232.0(m),

284.6(s),

232.0(s),

, 347.2U)

492.2 (m)

, 2B4.6U), 351.Ks), 423.6U), Ii47.9(s)

, 655.91s)

284.6(s), 492.2(s), 555.0(s)

232.0(s), 351.l(s), 423.6(s),

, 655.9(m)

. 512.3(s)

512.3(s), 859.5(5)

284.6(5), 436.Ks), 724.3(s)

347.2(5). 423.6(1»)

655.9(s)

. 351.Us)

, 555.0U)

a)The uncertainty in the f-ray intensities is estimated to be ±10 % for the strong peaks

(above 10 % relative intensity) and ±20 % for the weak ones.

The uncertainty in the Y*ray energies is estimated to be ±0.5 keV. s(«strong) indicates

that the peak is higher than 3c (standard deviations) of the background, m(*mediuai)

indicates 1-3o and w(-weak) less than 1o.

Table 1

Energies, re la t ive in tens i t ies and coincidences observed f o r y - r a y s

assigned to Ce.

r n IrEnergy^ (keV)

Rel. I n t . "lTtt)

Observed coincidences '

35.0

52.2

87.0

98.7101.0

106.3

133.7

H I .5

210.7

218.5

251.2

264.9

317.1

352.1

369.8

M5.9

503.1

-

5.2

2.7

12.6

* . *

0.321.8

13.1

18.3

*1.310.0

38.3

100.0

0.6

SA

5.56.2

317.Ks)

264.9(s)

264.9U)2i8.5(s)

251.2(s)

None

218.5U)

210.7(s)

1*1.5(s)

98 .7 (s ) , 133.7(s)

101.0(s)

52.2(5) , 87 .0U)

35.0(s)

None

98.7.J1), 106.3(m)None

None

a) The uncertainty in the T~ray intensities is estimated to be ±20 %,

The uncertainty in the f-ray energies is estimated to be ±0.5 keV.For contnents» see Table 1 .

Table 2

Energies, relative intensities and coincidences observed for y~rays

assigned to Ce.

J

rr-10. rtntrgy^ (keV)

92.9

178.«

198.9

25«.«

269.1

289.9

362.0

37«.«

«39.8

«52.3

«67.3

832.2

I19«.2

The uncertainty

del. int."

51.1

5.3

25.2

7.9100.0

19.3

6.2

«3.«

28.0

29.8

15.7

<3

in the Y'ray intensities


•9B.9(s). 269.Ms). 37«.«(s). 2$«.«(w), «39.8(s).

«52.3<«), 832.2(s)

198.9(i»). 289.9(i»)

178.Mm), 25«.«(M)

198.91»), 289.9<«0

92.9(s), *39.8{s), 832.2(s)

178.«(m), 25«.«(»)

«39.8(s), 832.2(5)

92.9(s)

92.9(sj. 269.l(s). 362.0(s)

92.9(s)

None

92.9(s). 269.1(s), 362.0(s)

None

is estimated to be ±20 %.

The uncertainty in the Y'ray energies is estimated to be ±0.5 keV.

For comments, see Table 1.

Table 3

Energies, relative intensities and coincidences observed for y-rays1/.7

assigned to 'Ce.

J

r H. '* rEnergyly (keV)

74.590.1.

98.5

105.2

116.8

121.2

130.1

168.3

191.7

195.7

233.7

273.8

269.7

291.8

325.0

332.7

3*7.2

369.3

374.4

390.6

399.7422.0

520.7

Rel. int."

14.8

75.9

29.5

18.3

72.2

3. 44.3

7.740.0

6.0

32.6

29.0

100.0

45.2

5.0

8.914.0

4.0

3-0

6.1

22.52.0


EMpected transition

IO5.2(s), I95.7(s). 269.7(s), 325.0(s), 374.4(s)

I91.7(n>). 291.8(s), 269.7(s), 325.0(s), 369.3(s),

422.0(s)

90.4(s), I95.7(s), 273.8(s), 325.0(s)

273.8(s)

233.7(m), 269.7(s), 347.2(s), 399.7(s)

195.7M, 269.7(s), 291.8(s). 390.6(w)

Not gated

98.5(s), 332.7(s)

I95.7(s), 273.8(s), 325.0(s)

269.7W

H6.B(s)

121.2(s), 130.1(m), 233.7(m)

98.5U). 130.1(5)

90.4(s), 98.5(s), 105.2(s), 195.7(s)

Not gated

121.2(s)

98.5(s)

90.4(s)

Not gated

Not gated

98.5ts)None

b)

The uncertainty in the y ray intensities is estimated to be ±20 %.

The uncertainty in the.y'ay energies is estimated to be ±0„5 keV.

For coRflients, see Table 1.

Table

Energies, relative intensities and coincidences observed for y rays

assigned to Ce.

J

r 12.

0.3 M HDEHP in kerosene

Gas jetfrom target

1MHNO»

1 M HNOo0.4 M H2SO40.2 M K2Cr2O7

1 MHNO30.1 M H2S04

0.05 M K2Cr2O?

1M0.05 M005 M

Figure 1

Flow sheet showing the chemical system used for the isolation of

Ce isotopes. M = mixer, Dg = degassing unit, C1-C3 = mixer-

centrifugal separator units, E = extraction column, D = detectors,

FP = fission products

r13

»12

2 "

uizz<zoEIU

a.(0HZ3oo

46

ai ai ai ai c ât« ai ai a* »

O CM co 00 v> •— cO* O* OR Oft O <M «

46 Cr 00 ^ ) OO

i n lOCMCM CO CM CM

at

oi n

a* a» <u ai ai a» a»uouva co corô C7» O

ai(j

*n

S

2 0 0 4 0 0 6 0 0 8 0 0

^ J J L K ^

1200 1400 1600

CHANNEL NUMBER

1800

Figure 2

y-ray singles spectrum of neutron-rich Ce isotopes. The spectrum

was recorded on-line during 30 minutes.

rr

14S

<p 0> 0,1

2 5 SR ? ig

]

w w ^ «? ^ w

10 M ^ _0) oj ° °? r: Pm r * ffl <o ru» in CM « co «

1» T}- >* CM

<3* S" 'S1

ö Ä ÔO CM h.iri oi Sm o ou> ^ CM

(<

3> -55» ®O N ?—> >—< O

2 S 3

r«.

2

l

1210.7

859.6

787.1

555.0

351.0347.4

62.7

0.0145, Pr

Figure 3

Partial decay scheme of Ce. The scheme is based on the


r 15.

146Ce \ i

itsII)

m"

tri

S_CO m

Äm

SiCM

U)00

CMÖCM

503.1

352.1

® 'S1

g, sU> "^ f !

5 § ^r* o O

W 00 g- Kro o> o oo

OCM trim co

141.5133.7101.087.0

3500.0

146,Pr

F igu re -:•

P a r t i a l decay scheme o f Ce. The scheme i s based on the


16.

T1/2-56s

CM

S

5.7

832

1194.2

000)CO

©OSCM

COCMU><*

en

CM

090)CM

CO(fr

CO

CO

COIX)

«t00

CM(O

oCM<OCO

o6o

^<n(OCM

.3)

enm00CM

CMinCM

0)CO0)

.1)

m

enCMen

801.8

544.2

467 3

362.0

289 8

92 9

0.0147 Pr

Figure 5

147Part ia l decay scheme of Ce„ The scheme is based on the

coincidence: isted in Table 3«

r148 C e \ ßT1/2 = '

o o o i n ^ N ^ ' O O i ' i o ' p p p p o r ^ i o O 1 * ) W 'S "» 'Si n ' « « « » « « » » « ' ' ^ S S g ^ ^ d o i c\i cb in «»

f pv ^ O r^ O ^ C5 N 00 *^ ^ ® ^ T ^ f*5 ^ M ID ^ ^ ^ ^

623.5

520 7468.6464. S

390.6

289.9

195.8

121. Z116.898 590.4

0.0

148Pr

Fipure 6

Partial decay scheme of Ce. The scheme is based on the

coincidences listed in Table 4„

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